In a skillet, cook sausage and onion until the sausage is browned and the onion is tender; drain. Remove from the heat; stir in 1 cup cheese, spinach and mushrooms. Transer to a greased 13-in. X 9-in. X 2-in. Baking dish. In a bowl, beat eggs. Add cream and nutmeg; mix well. Pour over sausage mixture. Bake, uncovered, at 350 for 35-40 minutes or until a knife inserted near the center comes out clean. Sprinkle with remaining cheese. Let stand 5 minutes before cutting.