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The mushrooms are the first thing I watch in the skillet: they start out crowded and glossy, then turn brown as their liquid cooks away. That’s when this vegan green bean casserole gets its deep, savory base instead of tasting like green beans under a thin white sauce. Fresh beans keep their bite, and a thick oat milk and vegetable broth sauce coats every piece.

I blanch the beans before they go into the oven, which gives them a head start without leaving them soft after baking. The sauce comes together in the same skillet, so the onion and browned mushrooms carry their flavor into every spoonful. It’s a generous 9×13-inch casserole that serves eight, with a crisp onion topping added at the very end.

The finished dish has tender green beans, a spoon-coating mushroom sauce, and a golden top that crackles when you serve it. That contrast is what makes this one feel like a proper casserole, not just vegetables baked in sauce.

Why the Crispy Onions Go on After the First Bake

A casserole topping has two jobs: it needs to stay crisp, and it needs time to turn golden without scorching. Vegan crispy fried onions are already cooked, so they don’t need the full time in the oven that the beans and sauce do. Bake the casserole first, then scatter the onions over the bubbling surface for the final few minutes. They warm through and deepen in color while keeping more of their crunch.

Adding them at the beginning changes the texture. The onions sit against steam from the sauce for the entire bake, and their delicate crispness can disappear before the casserole is ready. Keeping them back also makes it easier to spread them evenly across the top instead of stirring them into the beans. When you take the dish out, look for golden edges on the onions and bubbling sauce around the sides; the middle will settle as it rests.

What Gives the Mushroom Sauce Its Savory, Creamy Body

• Fresh green beans — Fresh beans hold their shape through blanching and baking, so the casserole keeps some bite. Trim them and cut them into even pieces for consistent cooking. Yellow wax beans work as a close swap; trim and cut them the same way.
• Cremini mushrooms — Their earthy taste gives the sauce its savory backbone, and browning them builds more depth than adding them straight to the liquid. They don’t need to be expensive; choose firm mushrooms without damp or slimy spots. White button mushrooms work in the same quantity for a milder result.
• All-purpose flour — The flour thickens the broth and oat milk into a sauce that clings to the beans. Cooking it briefly in the skillet before adding liquid keeps the finished sauce from tasting floury. A gluten-free 1-to-1 baking flour blend can replace it; see the adaptation below.
• Unsweetened plain oat milk — It brings creaminess without dairy and keeps the mushroom flavor in front. Check that it’s plain and unsweetened, since vanilla or sweetened milk would make the sauce taste out of place. Unsweetened plain soy milk is another workable option.
• Soy sauce and vegetable broth — Soy sauce adds a concentrated savory note, while broth supplies most of the sauce’s liquid. A similar amount of tamari can stand in for the soy sauce. Use a broth you enjoy sipping; a very salty broth can make the finished sauce taste saltier than intended.
• Vegan crispy fried onions — These are what make the top crunchy without adding a separate topping mixture. Buy a bag that’s fresh and crisp; a stale topping won’t regain its snap in the oven. Keep them out of the casserole until the final bake.

From Crisp-Tender Beans to a Bubbling Casserole

Blanch the Beans Without Softening Them

Heat the oven to 375°F and grease a 9×13-inch baking dish with olive oil. Bring a large pot of water to a boil, add the trimmed, cut green beans, and cook for 4 to 5 minutes. They should look bright green and bend slightly while still feeling crisp-tender. Drain, rinse under cold water, and drain well; the rinse stops the cooking so the oven doesn’t push the beans past tender.

Brown the Mushrooms Before Building the Sauce

Warm the olive oil in a 12-inch skillet over medium heat, then add the finely chopped onion and sliced cremini mushrooms. Cook for 8 to 10 minutes, stirring occasionally, until the mushrooms are browned and their liquid has evaporated. If the pan still looks wet, keep cooking; the mushrooms need to brown rather than simmer in their own liquid. Stir in the minced garlic and cook for 30 seconds, just until fragrant.

Whisk Until the Sauce Coats a Spoon

Stir the flour into the mushroom mixture and cook for 1 minute, so it combines with the oil and vegetables. Gradually whisk in the vegetable broth and oat milk, working out any lumps as the liquid goes in. Stir in the soy sauce, kosher salt, and black pepper, then simmer for 4 to 6 minutes, stirring, until the sauce is thick enough to coat a spoon. If you pour in the liquid all at once, the flour is harder to whisk smooth.

Bake First, Then Add the Crunch

Put the blanched beans in the prepared dish, pour the mushroom sauce over them, and stir until everything is evenly coated. Bake for 20 to 25 minutes, until the sauce bubbles around the edges. Scatter the vegan crispy fried onions evenly over the top and bake for another 5 to 8 minutes, until golden. Rest the casserole for 5 minutes before serving so the sauce can settle around the beans.

Simple Swaps for a Different Table

Questions That Come Up While Making This Casserole

Vegan Green Bean Casserole

Vegan green bean casserole combines crisp-tender green beans with a creamy mushroom sauce, baked until bubbling and topped with golden crispy fried onions. This oven-baked side is made with oat milk and serves eight.

Print Recipe Pin Recipe

Prep Time 20 minutes mins

Cook Time 50 minutes mins

Resting 5 minutes mins

Total Time 1 hour hr 15 minutes mins

Servings: 8 servings

Course: Side Dish

Cuisine: American

Calories: 230

Ingredients Equipment Method Notes

Ingredients

  

For the casserole

• 1 tsp olive oil for greasing

For the green beans

• 2 lb green beans fresh, trimmed and cut into 2-inch pieces

For the mushroom sauce

• 3 tbsp olive oil
• 1 yellow onion medium, finely chopped
• 12 oz cremini mushrooms sliced
• 2 cloves garlic minced
• 0.25 cup all-purpose flour
• 2 cups vegetable broth
• 1 cup oat milk unsweetened plain
• 1 tbsp soy sauce
• 0.5 tsp kosher salt
• 0.25 tsp black pepper

For the topping

• 1.5 cups vegan crispy fried onions

Equipment

• 1 9x13-inch baking dish
• 1 12-inch skillet

Method

 

Prepare the casserole

1. Heat the oven to 375°F.
2. Grease a 9x13-inch baking dish with 1 teaspoon olive oil.

Blanch the green beans

1. Bring a large pot of water to a boil.
2. Add the green beans and cook for 4 to 5 minutes until bright green and crisp-tender.
3. Drain the green beans, rinse under cold water, and drain well.

Make the mushroom sauce

1. Heat 3 tablespoons olive oil in a 12-inch skillet over medium heat.
2. Add the onion and mushrooms, and cook for 8 to 10 minutes, stirring occasionally, until the mushrooms are browned and their liquid has evaporated.
3. Stir in the garlic and cook for 30 seconds.
4. Stir in the flour and cook for 1 minute.
5. Gradually whisk in the vegetable broth and oat milk until smooth.
6. Stir in the soy sauce, salt, and pepper.
7. Simmer for 4 to 6 minutes, stirring, until thick enough to coat a spoon.

Assemble and bake

1. Add the green beans to the prepared baking dish, pour the mushroom sauce over them, and stir until evenly coated.
2. Bake for 20 to 25 minutes until bubbling around the edges.
3. Scatter the crispy fried onions evenly over the top.
4. Bake for another 5 to 8 minutes until golden.
5. Rest the casserole for 5 minutes before serving.

Notes

Drain the green beans well so the sauce stays creamy. Refrigerate leftovers for 3 to 4 days; freezing is not recommended because the crispy onions lose their texture. For a gluten-free version, use gluten-free flour and crispy onions and replace the soy sauce with tamari.