melt butter over medium-high heat in a 12-inch cast iron skillet.
Add the mushrooms, salt and pepper and saute them until they start releasing their juices, 3-5 minutes.
Add the garlic and saute for one minute.
Add the flour and stir it until the mushrooms are fully coated.
Add the broth, 1/4 cup at a time, stirring constantly.
Simmer for 1 minute, then add cream and bring back to a simmer,
Add nutmeg and continue cooking until the sauce thickens, about 5 to 6 minutes, stirring frequently.
Add the cooked green beans to the sauce and stir well.
Place the mixture in a 9x13 baking dish and top with the crispy onions.
Bake for 15 minutes at 400 degrees.