1. Preheat your oven to 375°F (190°C). Blanch the green beans in boiling salted water for 5 minutes until bright green and tender-crisp. Transfer to an ice water bath, then drain and set aside.

2. In a large skillet, melt the butter over medium-high heat. Add the mushrooms and shallots and sauté until browned and softened, about 8-10 minutes. Add the garlic and cook for 1 more minute.

3. Sprinkle flour over the mixture, stir to combine, and cook for 1 minute. Gradually whisk in the milk and heavy cream until smooth.

4. Bring the sauce to a simmer and cook, stirring frequently, until thickened, about 5-7 minutes. Remove from heat and stir in salt, pepper, and nutmeg.

5. Combine the green beans, sauce, and 1 cup of crispy fried onions in a large bowl. Stir gently to coat evenly.

6. Transfer to a 9×13-inch baking dish and spread evenly. Bake for 20 minutes.

7. Sprinkle the remaining 1/2 cup of crispy onions on top and bake an additional 5-10 minutes, until golden and bubbly. Let rest before serving.