Quick Recipe Version (TL;DR)
Quick Ingredients
- 2 lb (907 g) fresh green beans, trimmed
- 1 1/2 tsp kosher salt, divided (plus more for blanching water)
- 3 tbsp unsalted butter
- 12 oz (340 g) cremini mushrooms, finely chopped
- 1 small yellow onion, finely diced (about 1 cup / 150 g)
- 2 cloves garlic, minced
- 1/4 cup (32 g) all-purpose flour
- 1 1/2 cups (360 ml) whole milk
- 1/2 cup (120 ml) low-sodium chicken or vegetable broth
- 2 tsp soy sauce
- 1/2 tsp freshly ground black pepper (plus more to taste)
- 1/4 cup (60 ml) heavy cream (optional, for extra richness)
- 2 cups (about 90 g) crispy fried onions (store-bought) or 1 cup (50 g) panko + 2 tbsp melted butter
Do This
- 1. Heat oven to 375°F (190°C). Grease a 9 x 13-inch baking dish.
- 2. Blanch green beans in salted boiling water 3 minutes; ice-bath 2 minutes; drain well.
- 3. Sauté mushrooms + onion in butter 8–10 minutes; add garlic 30 seconds.
- 4. Stir in flour 1 minute. Whisk in milk + broth; simmer 3–5 minutes until thick. Season with soy sauce, pepper, and salt.
- 5. Toss beans with sauce; spread in dish.
- 6. Bake 15 minutes; add fried onions (or buttered panko) and bake 8–10 minutes more until golden.
- 7. Rest 5 minutes, then serve hot.
Why You’ll Love This Recipe
- Classic comfort, upgraded: A creamy mushroom sauce with real sautéed mushrooms tastes like the nostalgic version, only fresher.
- Crisp-tender beans: A quick blanch keeps the green beans bright and pleasantly snappy, not mushy.
- Big savory flavor: Black pepper and a small splash of soy add depth without making it taste “soy sauce-y.”
- Choose your crunch: Go traditional with fried onions or use buttery panko for a homemade, golden topping.
Grocery List
- Produce: Fresh green beans, cremini mushrooms, yellow onion, garlic (optional: parsley or chives for garnish)
- Dairy: Unsalted butter, whole milk, heavy cream (optional)
- Pantry: All-purpose flour, low-sodium chicken or vegetable broth, soy sauce, kosher salt, black pepper, crispy fried onions or panko breadcrumbs
Full Ingredients
For the Green Beans
- 2 lb (907 g) fresh green beans, trimmed
- Kosher salt, for blanching water
- 1/2 tsp kosher salt (for seasoning the casserole)
For the Creamy Mushroom Sauce
- 3 tbsp (42 g) unsalted butter
- 12 oz (340 g) cremini mushrooms, finely chopped (about 4 heaping cups)
- 1 small yellow onion, finely diced (about 1 cup / 150 g)
- 2 cloves garlic, minced
- 1/4 cup (32 g) all-purpose flour
- 1 1/2 cups (360 ml) whole milk
- 1/2 cup (120 ml) low-sodium chicken broth or vegetable broth
- 2 tsp soy sauce
- 1/2 tsp freshly ground black pepper (plus more to taste)
- 1 tsp kosher salt (or to taste; start with 1 tsp and adjust after adding soy)
- 1/4 cup (60 ml) heavy cream (optional, for a richer finish)
For the Crunchy Topping (Choose One)
- Classic fried onion topping: 2 cups (about 90 g) crispy fried onions
- Buttery panko topping: 1 cup (50 g) panko breadcrumbs + 2 tbsp (28 g) unsalted butter, melted + 1/4 tsp kosher salt
Optional Garnish
- 1 tbsp finely chopped fresh parsley or chives
- Extra freshly ground black pepper
Step-by-Step Instructions
Step 1: Preheat the oven and prep your baking dish
Arrange a rack in the middle of the oven and preheat to 375°F (190°C). Lightly grease a 9 x 13-inch baking dish with butter.
Set out a large bowl of ice water near the stove for the green beans (this helps lock in that bright green color and crisp-tender bite).
Step 2: Blanch the green beans for crisp-tender texture
Bring a large pot of water to a boil and season it generously with salt (aim for “salty like the sea”). Add the trimmed green beans and boil for 3 minutes.
Immediately drain the beans and transfer them to the ice water bath for 2 minutes. Drain again very well. (Extra water can thin your sauce, so shake the colander and blot the beans with a clean towel if needed.)
Step 3: Sauté mushrooms and onions until deeply flavorful
In a large skillet or Dutch oven over medium-high heat, melt 3 tbsp butter. Add the chopped mushrooms and diced onion.
Cook, stirring occasionally, for 8–10 minutes, until the mushrooms have released their liquid and most of it has cooked off. You’re looking for a slightly browned, savory base (this keeps the casserole from tasting watery).
Add the minced garlic and cook for 30 seconds, just until fragrant.
Step 4: Build the creamy mushroom sauce (and season it right)
Sprinkle 1/4 cup flour over the mushroom mixture and stir constantly for 1 minute. This cooks off the raw flour taste and helps your sauce thicken smoothly.
Slowly whisk in the 1 1/2 cups milk and 1/2 cup broth. Bring to a gentle simmer, whisking or stirring frequently, and cook for 3–5 minutes, until the sauce thickens enough to coat a spoon.
Stir in 2 tsp soy sauce, 1 tsp kosher salt, and 1/2 tsp black pepper. If using, stir in 1/4 cup heavy cream. Taste and adjust: you may want an extra pinch of salt or a few more turns of black pepper.
Step 5: Combine beans and sauce, then assemble
Add the well-drained green beans to the skillet and toss until evenly coated in the creamy mushroom sauce.
Spoon everything into the prepared baking dish and spread into an even layer, reaching into the corners so it bakes evenly.
Step 6: Bake until bubbly
Bake uncovered at 375°F (190°C) for 15 minutes, until the casserole is hot and bubbling around the edges.
Step 7: Add the crunchy topping and finish baking
Carefully remove the casserole from the oven. Add your topping of choice:
If using fried onions: Sprinkle 2 cups evenly over the top in a thick blanket.
If using buttery panko: Mix 1 cup panko with 2 tbsp melted butter and 1/4 tsp salt, then sprinkle evenly over the top.
Return to the oven and bake for 8–10 minutes, until the topping is crisp and golden brown.
Step 8: Rest briefly, garnish, and serve
Let the casserole rest for 5 minutes before serving. This helps the sauce set up so it stays creamy rather than runny.
If you like, finish with chopped parsley or chives and a little extra black pepper. Serve hot.
Pro Tips
- Drain the beans well: Water clinging to blanched beans can thin the sauce. A quick towel-blot makes a big difference.
- Cook off mushroom moisture: Sauté until the pan looks mostly dry before adding flour. This concentrates flavor and prevents a watery casserole.
- Season after soy: Soy sauce brings salt and savory depth. Add it before final salting so you don’t overdo it.
- Keep topping crunchy: Add fried onions (or panko) only for the final bake so they stay crisp, not soggy.
- Thicker sauce = better bake: Your sauce should coat a spoon before you combine it with beans; it will loosen slightly in the oven.
Variations
- Extra-savory version: Add 1/2 tsp Worcestershire sauce along with the soy for even deeper umami.
- Cheesy (still classic-ish): Stir 3/4 cup (75 g) shredded sharp cheddar into the sauce before baking.
- All-panko topping with herbs: Mix panko with melted butter plus 1 tbsp chopped parsley and 1/4 tsp garlic powder.
Storage & Make-Ahead
Refrigerate: Cool completely, then cover and refrigerate for up to 4 days.
Reheat: For best texture, reheat in a 350°F (175°C) oven for 15–20 minutes until hot. If the topping needs re-crisping, add a small handful of fresh fried onions (or a sprinkle of panko) for the last 5 minutes.
Make-ahead (best method): Assemble the casserole without the topping, cover, and refrigerate up to 24 hours. Bake at 375°F (190°C) for 20 minutes, add topping, then bake 8–10 minutes more.
Freezing: Cream sauces can separate after freezing, and green beans soften. If you must freeze, freeze the baked casserole without topping for up to 1 month, thaw overnight in the fridge, reheat at 350°F (175°C), and add fresh topping at the end.
Nutrition (per serving)
Approximate (using fried onion topping, 8 servings): 280 calories, 16 g fat, 26 g carbs, 6 g protein, 5 g fiber, 720 mg sodium. Values vary by brand and exact ingredients.
