Step 1: Prepare the green beans. Bring a large pot of salted water to a boil. Add the trimmed fresh green beans and blanch for 3-5 minutes, or until tender-crisp and bright green. Immediately drain and transfer to an ice bath to stop the cooking. Once cooled, drain thoroughly and pat dry.
Step 2: Make the creamy mushroom sauce. In a large skillet or Dutch oven, melt the butter over medium heat. Add the chopped shallot and cook until softened, about 3-5 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 3: Add the sliced fresh mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms release their liquid and start to brown, about 5-7 minutes.
Step 4: Sprinkle the flour over the mushroom mixture and stir well. Cook for 1 minute, stirring constantly, to cook out the raw flour taste.
Step 5: Gradually whisk in the chicken or vegetable broth, scraping up any browned bits from the bottom of the pan. Bring the mixture to a simmer, stirring constantly, until it thickens slightly.
Step 6: Stir in the heavy cream and Worcestershire sauce. Season with salt, black pepper, and a pinch of nutmeg. Taste and adjust seasonings as needed. The sauce should be rich and flavorful.
Step 7: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 8: In a large bowl, combine the blanched green beans with about two-thirds of the creamy mushroom sauce. Stir gently to ensure the beans are evenly coated.
Step 9: Pour the green bean mixture into the prepared baking dish. Sprinkle half of the crispy fried onions evenly over the top of the casserole.
Step 10: Bake for 20-25 minutes, or until the casserole is bubbly around the edges and heated through.
Step 11: Remove the casserole from the oven. Sprinkle the remaining crispy fried onions over the top.
Step 12: Return to the oven and bake for another 5-10 minutes, or until the fried onions are golden brown and extra crispy. Keep a close eye on them to prevent burning. Let rest for a few minutes before serving hot.