Cook spaghetti according to package directions; drain. In a small bowl, combine the pasta, egg and Parmesan cheese. Press onto the bottom and up the sides of a 7-in. pie plate or shallow 2-cup round baking dish coated with cooking spray; set aside.
Crumble sausage into a small skillet; add onion. Cook over medium heat until meat is no longer pink; drain. Stir in the tomato sauce, sour cream and Italian seasoning. Spoon into crust.
Bake, uncovered, at 350° for 15-20 minutes or until heated through. Sprinkle with mozzarella cheese; bake 2-3 minutes longer or until cheese is melted. Serve immediately.