In a large saucepan, cook sausage and onion until meat is brown and onion is tender. Drain off fat. Stir in tomatoes, tomato sauce, and Italian seasoning. Bring to boiling; reduce heat. Simmer, uncovered, for 15 to 20 minutes or until sauce reaches desired consistency.
Meanwhile, cook noodles according to package directions; drain. Rinse with cold water; drain again. For filling: In a medium bowl, combine egg, ricotta cheese, and 1/4 cup of the Parmesan cheese. Stir in spinach.
To assemble, in an ungreased 12x7-1/2x2-inch baking pan, arrange three of the cooked noodles. Spread with half of the filling. Top with half of the meat sauce and half of the mozzarella cheese. Repeat layering noodles, filling, meat sauce, and mozzarella cheese. Sprinkle with the remaining Parmesan cheese.
Bake, uncovered, about 30 minutes or until heated through. Let stand for 20 minutes before serving. Makes 8 servings.