Cook manicotti to desired doneness as directed on package. Drain; place in cold water.
Meanwhile, heat oven to 350°F. In large skillet, brown sausage, onion and mushrooms. Remove from skillet; drain on paper towels. In large bowl, combine sausage mixture, turkey, egg yolks, spinach, mozzarella and 1/2 cup Parmesan cheese; mix well.**
Drain manicotti; fill each with sausage mixture. Pour 1/2 the spaghetti sauce into ungreased 13x9-inch (3-quart) baking dish. Place filled manicotti side by side over sauce in dish.
Melt margarine in medium saucepan over medium heat. Stir in flour; cook until smooth and bubbly, stirring constantly. Gradually stir in whipping cream; cook until slightly thickened, stirring constantly. Stir in 1/3 cup Parmesan cheese, garlic powder and nutmeg; blend well. Pour white sauce over manicotti in baking dish. Pour remaining spaghetti sauce over top.
Bake at 350°F. for 35 to 40 minutes or until bubbly.