In a large skillet, saute mushrooms and onions in butter until slighly brown. Add the beef and saute to brown. Salt and pepper to taste. Squeeze spinach dry, add to skillet, and add a pinch of nutmeg. Pour in eggs, and cheese if desired, stir, and cook until lightly set. Turn and cook 1 to 2 minutes or until firm. Serve with a green salad. Serves 4 to 6.