1. Wash and trim the ends of the green beans and cut into 2-inch pieces

2. Bring a large pot of salted water to a boil, add the green beans, and cook for 3-4 minutes until crisp-tender

3. Transfer the blanched beans to an ice water bath to stop cooking, then drain and pat dry

4. In a skillet, melt butter over medium heat and cook chopped onion until translucent

5. Add sliced mushrooms and cook until they release their juices and turn golden

6. Stir in minced garlic and cook for 30 seconds until fragrant

7. Sprinkle flour over the vegetables and stir for one minute to form a roux

8. Gradually whisk in milk, stirring until the sauce thickens and coats the back of a spoon

9. Season the sauce with salt, pepper, and nutmeg, then add the drained green beans and stir to coat

10. Butter a glass baking dish and transfer the green bean mixture into it

11. Top the casserole evenly with crispy fried onions

12. Bake in a preheated oven at 350°F for 25-30 minutes until the topping is golden and the sauce is bubbly

13. Let the casserole rest for 5-10 minutes before serving