In a medium sized saucepan, combine chicken breasts and onion with water to cover. Cover and poach chicken over medium-low heat until white throughout (about 20 minutes).
Meanwhile, melt margarine in a frying pan. Add mushrooms and garlic and cook until tender, about 5 minutes.
Preheat broiler. Arrange fresh spinach evenly in an 8-inch square baking dish. Sprinkle with nutmeg, salt and pepper to taste. Arrange chicken in a single layer over fresh spinach. Top with mushrooms. Sprinkle with cheese. Broil for 1-2 minutes until cheese is melted.