Preheat your oven to 375℉ and cook the shells according to package directions.
Drain the pasta shells and set aside while making the cheese mixture.
Stir together in a large bowl, the ricotta, 1 cup of mozzarella, parmesan, parsley, garlic, onion, and salt.
Spread 1 ½ cups of marinara sauce on the bottom of a 9-inch by 13-inch baking dish, and ½ cup of sauce on the bottom of the 8-inch by 8-inch baking dish.
Fill the shells with 1 to 2 tablespoons of the cheese mixture each, and then place in the baking dishes. Both dishes should be completely full.
Spoon another 1 ½ cups of sauce over the shells in the 9-inch by 13-inch dish, and another ½ cup of sauce over the 8-inch by 8-inch dish.
Sprinkle ¾ cup of shredded mozzarella over the shells in the larger dish, and ¼ cup mozzarella over the shells in the smaller dish.
Place foil over both baking dishes and bake for 20 minutes.
Carefully remove the foil and bake for another 10 minutes. Serve while still hot.