• Preheat the oven to 350°F.

  Prepare the Green Beans

  Use fresh or frozen — whichever fits your day.

  Fresh: Blanch in salted boiling water 5 minutes, then plunge into ice water and drain.

  Frozen: Thaw and drain. No blanching needed.

  Sauté the Vegetables

  In a 12-inch cast iron skillet (or large skillet), melt 4 tablespoons butter over medium heat.

  Add the diced onion and cook 4–5 minutes until softened.

  Add mushrooms and cook another 3–4 minutes.

  Stir in garlic and cook 1 minute.

  Build the Sauce

  Add the remaining 4 tablespoons butter to the pan.

  Sprinkle in the flour and whisk to form a paste. Cook 1–2 minutes.

  Slowly whisk in cream (or half-and-half), stock, Worcestershire, hot sauce, nutmeg, salt, and pepper.

  Simmer 4–5 minutes, stirring, until thick and silky. Taste and adjust seasoning.

  Combine

  Add the prepared green beans to the skillet and stir to coat.

  If not using cast iron, transfer the mixture to a buttered 9×9 baking dish.

  Top and Bake

  Cover the top evenly with the fried onions.

  Bake 30 minutes, until bubbly and the onions are golden.

Make-Ahead

• Assemble the casserole without the onion topping.

  Cover and refrigerate up to 24 hours.

  Bake, then add onions during the last few minutes to keep them crisp.