Preheat oven to 400 degrees F (200 degrees C).

Heat the olive oil in a saucepan over medium heat. Stir in the onion, and cook until translucent, about two minutes. Stir in the tomato paste, tomato sauce, stewed tomatoes, Italian seasoning, basil, oregano, mushrooms, and 2 cups of the provolone. Cook until the cheese has melted, stirring occasionally. While the sauce is cooking, place the frozen shells and meatballs in the bottom of a 9x13 inch glass dish. Pour the sauce over the shells, then sprinkle on the remaining 2 cups of provolone. Cover the top of the dish with foil.

Bake in the preheated oven until the pasta has cooked through, and the cheese has melted, about 30 minutes.