Bring 1 gallon of water and 2 tbs salt to a boil. Add the green beans and cook for 5 minutes, then drain and plunge in ice water. Drain again and set aside.
Melt 2 tbs butter over medium-high heat.
Add the mushrooms then salt and pepper to taste. Stirring occasionally, cook until the mushrooms begin to give up some of their liquid, about 4-5 minutes.
Add the garlic and nutmeg, then continue to cook another 1-2 minutes.
Sift or sprinkle in flour. Stirring constantly, cook for another minute.
Add broth and let simmer for 1 minute. Decrease the heat to medium-low and add the half-and-half. Stirring occasionally, cook until mixture thickens, about 8-10 minutes.
Remove from heat and add the green beans. Stir to coat and combine.
Transfer to oven-safe container, top with crunchy onions, and bake until bubbly, about 15-20 minutes.