Cook spaghetti according to package directions; drain. In a small bowl, combine the egg, Parmesan cheese and spaghetti. Press spaghetti mixture onto the bottom and up sides of a greased shallow 2-cup baking dish or 7-in. pie plate. Spread with sour cream.
Crumble turkey into a skillet; add the pepper, onion and butter. Cook over medium heat until meat is no longer pink; drain. Stir in the tomato sauce, garlic salt, oregano, salt and pepper. Spoon into spaghetti crust. Sprinkle with mozzarella cheese. Cover edges loosely with foil.
Bake at 350° for 15-20 minutes or until heated through and cheese is melted. Serve immediately.