Instructions:
- Blanch the beans: Bring salted water to a rolling boil. Add green beans and cook for 3-4 minutes until bright green and slightly tender (al dente). Immediately drain and plunge into an ice bath to stop cooking. Drain well.
- Sauté Aromatics: In a large, deep skillet or Dutch oven, melt 2 Tbsp of butter over medium heat. Sauté diced onion until softened (about 5 minutes). Add sliced mushrooms and thyme; cook until mushrooms have released their moisture and started to brown (about 8–10 minutes).
- Make the Roux: Push vegetables to one side of the skillet. Add the remaining 2 Tbsp of butter to the empty space. Once melted, whisk in the flour. Cook, stirring constantly, for 1–2 minutes until the mixture smells slightly nutty.
- Deglaze & Build Sauce: If using, pour in the sherry/wine and scrape up any browned bits. Let it bubble for 1 minute. Gradually whisk in the warm broth until smooth. Slowly whisk in the milk/half-and-half, stirring continuously until the sauce thickens enough to coat the back of a spoon (about 5–8 minutes).
- Season the Sauce: Remove from heat. Stir in Dijon mustard, Worcestershire sauce, salt (cautiously), and black pepper. Stir in 1/2 cup of the crispy fried onions and the optional shredded cheese until melted.
- Combine: Gently fold the blanched green beans into the mushroom sauce until evenly coated. Transfer the mixture to a prepared 9x13 inch baking dish.
- Top & Bake: Sprinkle the remaining 1 cup of crispy fried onions evenly over the top. Bake at 375°F (190°C) for 15–20 minutes, or until the casserole is bubbly and the topping is deeply golden brown.
- Rest: Let the casserole rest for 5–10 minutes before serving.