Crumble the sausage into a large skillet. Cook over medium heat until lightly browned.
Add the onion and cook until it is soft.
Add the crushed tomatoes, tomato paste, and Italian seasoning. Stir. Let the sauce come to a slow simmer.
Meanwhile, cook the ravioli per the package instructions. Drain well.
Preheat your oven to 350 F.
Spray a 9 x 11 baking dish with non-stick spray.
Spread 1 cup of the meat sauce over the bottom of the dish and spread out evenly. It'll be thin but try to get it everywhere.
Add the cooked ravioli, distributing it evenly around the pan. Do not overlap the pasta.
Add half of the cheese then another cup of sauce.
Add the last of the ravioli and sauce, followed by the remaining cheese.
Cover the dish in foil and bake for 20 minutes.
Remove the foil. Turn your broiler on and place the dish under the broiler for a few minutes (keeping an eye on it the entire time) until you get a little browning on top.