In a large skillet, melt butter over medium heat. Add mushrooms, salt and pepper and cook for 4 minutes, stirring occasionally. Add onion, garlic and nutmeg and continue to cook for an additional minute, until fragrant. Sprinkle flour over the mixture and stir to combine. Cook for 1-2 minutes. Stir in broth and bring to a simmer. Decrease the heat to medium-low and stir in evaporated milk. Cook for 5-7 minutes, stirring occasionally, until the mixture thickens.
Stir in green beans.