Cook fettuccini according to directions. Meanwhile in a 10 inch skillet, melt butter. Stir in garlic. Add mushrooms and onion. Cook over medium heat, stirring occasionally until soft (5 to 7 minutes). Stir in spinach and heat through 3 to 4 minutes. Drain fettuccine. Return fettuccine to saucepan; stir in spinach mixture. Remove to serving platter. Sprinkle with Parmesan cheese. Serves 4.