Preheat oven to 375 degrees F and lightly grease a 9×13" baking dish.
In a large skillet, cook Italian sausage and onion over medium heat until sausage is browned.
Stir in tomato sauce, garlic and Italian seasoning and simmer for a couple of minutes until onions are tender.
Spread ⅓ of the sauce into the bottom of a baking dish and top with half of the frozen ravioli. Top with 2 cups shredded mozzarella and another ⅓ of the sauce.
Top with the remaining ravioli, the last ⅓ of the sauce and remaining 2 cups cheese. Sprinkle with Parmesan.
Lightly grease a piece of tin foil and cover tightly. Bake for 30 minutes, or until ravioli are tender, then uncover and broil or brown the cheese as desired.