Fill a small saucepan with one inch of water. Bring to a boil, add peas, and cook until tender, about 5 minutes. Drain and set aside.
Melt butter in a medium saucepan over medium heat. Add mushrooms and onions, and cook for a few minutes, or until tender. Sprinkle flour over the mushrooms, and cook for 1 minute, stirring constantly. Gradually stir in cream, and season with salt, pepper and nutmeg. Cook, stirring until smooth and thick. Stir in peas, and remove from heat. Let stand for 5 minutes before serving.