Blanch the beans. Bring a large pot of salted water to a boil. Add green beans and cook 3–4 minutes until bright green and just tender. Drain and plunge into an ice bath to stop cooking. Drain well and pat dry.

Cook the mushrooms. In a large skillet, heat butter or oil over medium-high. Add mushrooms and cook, undisturbed, for 2 minutes to brown. Stir, then add a pinch of salt. Cook until they release moisture and it evaporates, 6–8 minutes total.

Add aromatics. Reduce heat to medium. Add shallot and cook 2 minutes until soft. Stir in garlic and thyme; cook 30 seconds until fragrant.

Build the sauce. Sprinkle flour over the mushrooms. Stir for 1 minute to coat. Slowly whisk in broth, scraping up any browned bits. Simmer 2 minutes until slightly thickened. Whisk in cream. Add soy sauce or Worcestershire, nutmeg if using, and season with salt and pepper. Simmer 3–5 minutes until the sauce coats a spoon.

Combine beans and sauce. Stir green beans into the skillet, coating evenly. Taste and adjust seasoning. The mixture should be well-seasoned since it will mellow a bit after chilling.

Transfer to dish. Grease a 9x13-inch baking dish (or similar). Spread the bean mixture evenly. Let cool to room temperature.

Prep the topping. In a bowl, combine French-fried onions with Parmesan and panko if using. Keep this mixture in an airtight container at room temp. Do not add to the casserole yet.

Cover and chill. Wrap the casserole tightly with foil or a lid and refrigerate up to 2 days. For longer storage, see freezing notes below.

Bake when ready. Preheat oven to 350°F (175°C). Uncover the casserole, stir once if needed, and bake for 20–25 minutes until hot and bubbling at the edges.

Add the topping. Sprinkle the onion mixture evenly over the hot casserole. Bake another 8–12 minutes until golden and crisp. Let rest 5–10 minutes before serving so it sets slightly.