If using fresh green beans, trim the ends and cut them into 1-inch pieces. Blanch them in boiling water for 3-4 minutes, then transfer them to an ice bath to stop the cooking process.
Melt butter in a large skillet over medium heat. Add chopped onion and garlic and sauté until softened. Add sliced mushrooms and cook until they release their moisture and start to brown. Sprinkle in flour and cook for 1 minute, then gradually whisk in vegetable broth and heavy cream. Bring to a simmer and cook until the sauce has thickened, about 5-7 minutes. Season with salt, pepper, and a pinch of nutmeg.
Preheat your oven to 350°F (175°C). In a large bowl, combine the blanched green beans and the mushroom sauce. Stir to coat. Pour the mixture into a greased 9×13 inch baking dish. Sprinkle the crispy fried onions evenly over the top.
Bake for 20-25 minutes, or until the casserole is bubbly and the onions are golden brown.
