Name: Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe
Prep time: 20 min
Cook time: 1 h
Total time: 1 h 20 min
Cuisine: American
Ingredients: 1½ pounds green beans, trimmed and halved; 16 ounces cremini mushrooms, sliced; 4 garlic cloves, minced; 2 tablespoons fresh thyme leaves; 1½ cups French fried onions, or homemade Crispy Baked Onions; ½ cup grated Parmesan cheese, plus more for sprinkling; 2 tablespoons extra-virgin olive oil; 2 tablespoons tamari; ¼ cup all-purpose flour; 3 cups milk, any kind; 1½ teaspoons Dijon mustard; ¼ teaspoon freshly ground black pepper

Jump to Recipe· 4.9 from 64 reviews

If you’re anything like me, you probably think of green bean casserole as a classic side dish that’s been around forever, but let me tell you about my twist that takes it to another level. This Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe is like your favorite comfort food, only way more luscious and fresh. The silky mushroom bechamel sauce makes everything rich and dreamy, and the crispy onions on top add that perfect crunch—trust me, once you try this version, the old canned soup version won’t cut it anymore. Stick around, and I’ll walk you through everything you need to get this dreamy, cozy side dish on your table.

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Why This Recipe Works

Creamy yet fresh: The mushroom bechamel sauce creates a silky, flavorful binder without heaviness.
Perfect texture contrast: Crispy onion topping brings delightful crunch that balances the soft beans and sauce.
Simple ingredients, elevated flavors: Using fresh mushrooms, tamari, and fresh thyme ups the taste game in an easy way.
Customizable and approachable: You can easily make it vegan or gluten-free while still capturing classic comfort.

Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping, green bean casserole, mushroom bechamel sauce, crispy onion topping, classic side dish - A white square tray filled with a single thin layer of crispy fried onion strings, showing a mix of pale golden and light brown colors, some pieces are curled and others are straight, with slight dark brown edges indicating toasted spots. The texture looks crunchy and scattered unevenly across the tray, resting on a white marbled surface. Photo taken with an iphone --ar 2:3 --v 7

Ingredients & Why They Work

This Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe relies on fresh, basic ingredients that complement each other beautifully. The green beans stay crisp and bright thanks to blanching, and the mushroom bechamel sauce ties everything together with creamy umami goodness—plus that salty tang from Parmesan. When you’re picking ingredients, opt for firm green beans and good-quality mushrooms; it’s worth the small splurge for flavor.

Green beans: Fresh, firm beans will hold up better after blanching and baking without getting mushy.
Extra-virgin olive oil: Adds a fruity base for sautéing the mushrooms and brings subtle richness.
Cremini mushrooms: They have a deep, earthy flavor and meaty texture that intensifies the sauce.
Tamari: This gluten-free soy sauce ramps up umami without overpowering—don’t skip it.
Garlic cloves: Fresh minced garlic infuses aromatic depth in the sauce.
Fresh thyme leaves: Thyme’s woody, herbal aroma pairs perfectly with mushrooms and dairy.
All-purpose flour: Thickens the bechamel for that velvety texture.
Milk (any kind): Creates the creamy base—use whatever you have, dairy or plant-based.
Dijon mustard: Adds a subtle tang that brightens the sauce beautifully.
Parmesan cheese: Brings salty, nutty complexity; you can sprinkle extra on top for garnish.
Freshly ground black pepper: Freshly cracked pepper enhances overall flavor without overwhelming.
French fried onions or homemade crispy baked onions: This is the crunchy, golden crown on your casserole—homemade versions feel fancy but aren’t hard.

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Note: You’ll find the complete list of ingredients, along with their exact measurements, in the printable recipe card at the bottom of this post.

Tweak to Your Taste

I love making this casserole my own depending on what’s in season or who’s coming to dinner. It’s easy to swap out ingredients if you want it vegan or need a gluten-free option. When I’m serving a crowd with varied diets, this recipe bends beautifully without losing any of its magic.

Vegan variation: I omit the Parmesan and use plant-based milk, adding extra salt to make sure the bechamel stays flavorful and rich.
Make it gluten-free: Using tamari ensures no gluten, and swapping flour for a gluten-free blend or cornstarch works great to thicken the sauce.
Swap the onions: If you prefer, crispy shallots or homemade panko crumbs work well for topping.
Add herbs: Fresh rosemary or sage can be added for a cozy herbal twist.

Step-by-Step: How I Make Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe

Step 1: Blanch the Green Beans to Keep Them Crisp and Vibrant

Start by bringing a large pot of salted water to a boil and prepping a bowl of ice water. Drop your trimmed and halved green beans into the boiling water for just 4 minutes—this softens them slightly while keeping that fresh snap. Immediately transfer the beans to the ice water bath to stop cooking so they stay bright green and don’t get mushy. Drain thoroughly and pat dry with a towel to keep excess moisture out of the casserole.

Step 2: Cook Mushrooms Then Build Your Mushroom Bechamel Sauce

Heat extra-virgin olive oil in a large skillet over medium heat. Add the sliced cremini mushrooms and let them soften, stirring occasionally, until their liquid cooks off and they start to brown—about 8 to 10 minutes. Then stir in tamari, garlic, and fresh thyme. Sprinkle the flour over the mixture and stir well to coat everything. Slowly add the milk with Dijon mustard, whisking carefully to avoid lumps. Simmer the sauce gently for 15 to 20 minutes, stirring frequently until thick and creamy. Finish by stirring in the Parmesan and freshly cracked black pepper. This step is where the magic happens—don’t rush it!

Step 3: Assemble and Bake Your Dreamy Casserole

Spread one-third of the mushroom bechamel sauce on the bottom of your greased 9×13 baking dish. Layer the blanched green beans evenly over the sauce, then pour over the remaining sauce to cover all the beans. Top the casserole generously with crispy fried onions—homemade or store-bought, both work beautifully. Cover the dish and bake at 400°F for 10 minutes, then uncover and bake for an additional 10 minutes to let the topping crisp up even more. If the onions start browning too fast, just pop the lid back on to avoid burning. Let it cool slightly before serving, and sprinkle a little extra Parmesan if you like.

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Pro Tips for Making Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe

Use fresh green beans: They hold their texture better than frozen and keep the casserole from getting watery.
Don’t skip the blanch and ice bath: This locks in color and texture so the beans don’t overcook in the oven.
Simmer the bechamel low and slow: Rushing this step risks lumps and less flavor development.
Crispy onions last-minute: Add them just before baking so they stay extra crunchy instead of soggy.

How to Serve Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe

Garnishes

I usually garnish with a sprinkle of freshly grated Parmesan cheese and a few thyme sprigs to echo the flavors inside. Sometimes I add a tiny drizzle of good-quality olive oil over the onions just before serving to add a silky sheen and extra richness. It’s simple but makes it feel more festive.

Side Dishes

This casserole pairs perfectly with roasted turkey or chicken, a classic for holiday dinners. I also love serving it alongside garlic mashed potatoes and cranberry sauce for a full comfort-food lineup. If you’re aiming for a vegan meal, a hearty lentil loaf or mushroom wellington complements it beautifully.

Creative Ways to Present

For a cozy dinner or Thanksgiving spread, I sometimes prepare this green bean casserole in individual ramekins topped with crispy onions and fresh herbs. It looks fancy yet feels homey. Another fun idea I’ve tried is layering it in a clear glass casserole dish to show off the vibrant green beans and creamy sauce — guests always ask for the recipe!

Make Ahead and Storage

Storing Leftovers

I store leftovers in an airtight container in the fridge for up to 3 days. The casserole holds up well but will lose a little crispness on top, so I usually add fresh crispy onions when reheating to reintroduce that crunch.

Freezing

I’ve frozen this casserole a couple times with success. Just be sure to freeze it before baking and wrap tightly. When ready, bake covered first to heat through, then uncover to crisp the topping. Avoid freezing the crispy onions on top; add those fresh after reheating.

Reheating

Reheat in the oven at 350°F until warm throughout—usually about 15-20 minutes. To revive the crispy onion topping, pop the dish under the broiler for just a minute or two, watching closely so it doesn’t burn.

FAQs

Can I use frozen green beans for this casserole?

Yes, you can use frozen green beans, but I recommend thawing and draining them well before assembly to avoid excess water in the casserole. Keep in mind that fresh green beans hold their texture better and give you a crisper bite.
How do I make the crispy onion topping from scratch?

Thinly slice yellow onions, toss with a bit of olive oil, flour, panko breadcrumbs, and salt, then bake at 450°F on parchment-lined trays for 25-35 minutes, stirring every 10 minutes, until golden and crunchy. This homemade version is fresher and not as salty as store-bought fried onions.
Is this casserole suitable for vegans?

Absolutely! Skip the Parmesan and use a plant-based milk to make your mushroom bechamel vegan. I also recommend adding an extra pinch or two of salt to keep the sauce flavorful without the cheese.
Can I prepare this casserole ahead of time?

Yes, you can assemble the casserole (without baking) a few hours ahead and keep it in the fridge. Add the crispy onions just before baking for optimal texture. This makes it perfect for holiday meal prep.
What can I substitute for tamari if I don’t have it?

You can use regular soy sauce, though it’s less gluten-free, or a splash of Worcestershire sauce. Just keep the quantity moderate to avoid overpowering the delicate mushroom flavor.

Final Thoughts

This Green Bean Casserole with Mushroom Bechamel and Crispy Onion Topping Recipe is one of those dishes that quickly became a family favorite in my home. There’s something so satisfying about combining fresh ingredients with classic comfort in a way that feels both elevated and cozy. Whether you’re making it for weekday dinners or your next holiday feast, I promise it’ll become your go-to green bean casserole recipe. Try it out and let me know how your family reacts—you might just make a new tradition!