Instructions:
- Blanch the beans: Bring a medium saucepan of salted water to a rolling boil. Add green beans and cook until bright green and tender-crisp (about 3-5 minutes). Immediately plunge into an ice bath to stop cooking. Drain well and pat dry. Set aside.
- Make Crispy Shallots: Toss sliced shallots with 1/2 cup flour and 1/2 tsp salt until lightly coated. Heat oil in a large, oven-safe skillet to 325°F (160°C). Fry shallots in batches until deep golden brown and crisp (about 3-5 minutes). Remove immediately with a slotted spoon, drain on paper towels, and sprinkle lightly with salt.
- Sauté Aromatics: In the same skillet, melt 2 Tbsp of butter over medium heat. Add diced onion and mushrooms; sauté until deeply browned and moisture has evaporated (about 8-10 minutes). Stir in thyme.
- Make the Roux: Push the mushroom mixture to one side. Add the remaining 2 Tbsp of butter to the empty side. Once melted, whisk in the 1/4 cup of flour until a smooth paste (roux) forms. Cook for 1 minute until nutty smelling.
- Deglaze & Thicken: Slowly whisk in the sherry/wine to deglaze the pan. Gradually whisk in the broth until smooth, followed by the milk/half-and-half. Bring to a simmer, stirring constantly until the sauce thickly coats the back of a spoon.
- Flavour the Sauce: Remove from heat. Stir in Worcestershire sauce, Dijon mustard, and black pepper. Taste and adjust seasoning.
- Combine: Gently fold the blanched green beans into the mushroom sauce until fully coated. Pour the mixture into a 2-quart casserole dish.
- Top & Bake: Sprinkle evenly with the optional Gruyère cheese. Bake at 375°F (190°C) for 20-25 minutes, or until bubbling hot and lightly browned on top.
- Finish & Serve: Let rest for 5 minutes after removing from the oven. Top generously with the reserved crispy shallots just before serving to maintain crunch.