Ingredients
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Fresh Green Beans (1 lb): Trimmed and sliced into bite-sized, 1-inch pieces.
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Mushrooms (8 oz): Freshly sliced to create that earthy, savory foundation.
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Small Yellow Onion: Finely diced for the crispy topping.
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Garlic (2 cloves): Minced to add a fragrant aromatic punch.
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Butter (3 tbsp): Divided for sautéing and building the roux.
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All-Purpose Flour (2 tbsp): To thicken the sauce into a velvety coating.
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Chicken Broth (1 cup): Use vegetable broth for a vegetarian-friendly version.
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Whole Milk (1/2 cup): For a smooth finish (or swap for half-and-half if you’re craving extra indulgence).
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Sea Salt (1 1/4 tsp): Divided to season both the onions and the sauce.
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Panko Breadcrumbs (1/2 cup): Provides a superior, airy crunch.
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Grated Parmesan (1/4 cup): Adds a salty, nutty depth to the crust.
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Cooking Oil Spray: Just a light mist to ensure a perfect golden brown in the oven.
Instructions
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Steam the Beans: Set a steamer pot over boiling water. Steam the green beans for about 8 minutes until they are fork-tender. Meanwhile, preheat your oven to 400°F.
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Prep the Crunchy Topping: In a large sauce pot, melt 1 tablespoon of butter. Sauté the diced onions with 1/4 teaspoon of salt for 4 minutes. Transfer the onions to a small bowl and toss with panko and parmesan cheese until well mixed; set this aside.
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Sauté the Mushrooms: Using the same pot, melt the remaining 2 tablespoons of butter. Add the mushrooms and 1 teaspoon of salt, sautéing for 4 minutes. Stir in the minced garlic and cook for 1 more minute until fragrant.
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Create the Creamy Sauce: Sprinkle the flour over the mushroom mixture, stirring for 30 seconds to coat everything evenly. Pour in the chicken broth and milk. Bring the mixture to a simmer and cook for 8–10 minutes.
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Pro Tip: The sauce is ready when it passes the “track test”—if you run a spoon through it, the path should stay visible for a moment before filling back in.
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Assemble & Bake: Fold the steamed green beans into the sauce, then pour the mixture into a greased 8×8-inch baking dish.
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The Final Touch: Evenly sprinkle your onion-panko mixture over the top and give it a light mist of cooking oil. Bake for 25 minutes or until the topping is beautifully golden brown.
Notes
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Make-Ahead Strategy: Save time on the big day by prepping the components up to 24 hours in advance. Store the green bean and mushroom mixture separately from the crispy onion topping. Simply assemble and bake whenever you’re ready for dinner!
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Storage Instructions: Have leftovers? Keep the casserole covered in the fridge for up to 3 days. For the best texture, reheat in the oven at 350°F—this helps the topping regain its crunch better than a microwave would.
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For the Ultimate Crunch: If you prefer an extra-crispy finish, switch the oven to broil for the last 1–2 minutes of baking. Watch it closely, though—it can go from golden to burnt in a heartbeat!
