Bring a large pot of water to a boil and season well with salt. Blanch green beans for 5 minutes and immediately remove to a cold bowl of water to stop the cooking. Drain after a minute or two and set aside.
Meanwhile, melt the butter in a large skillet over medium-high heat. Add mushrooms, salt, pepper and cook until mushrooms begin to sweat.
Add garlic and nutmeg and continue to cook for an additional 2 minutes.
Sprinkle the entire mixture with flour and stir until evenly distributed.
Add the broth and allow to simmer for a minute. Add cream and cook on low until mixture thickens (~ 5 minutes).
Remove from the heat and place in a casserole dish. Mix in ½ cup of the fried onions.
Bake for 15 minutes. Remove from oven and garnish top of casserole with the additional ½ cup onions.
Bake for an additional 5 minutes until sides are bubbly and onions are golden brown.