Bring a large pot of salted water to a boil, and cook the giant pasta shells according to package instructions. Drain and place them under running cold water to separate them. Reserve. Preheat the oven to 350 F.
Make the sauce by placing the store-bought marinara, Italian seasoning, garlic powder, and ground cinnamon in a medium-sized pot. Bring it to a boil, reduce the heat and simmer for 10 mins.
While the sauce is simmering, make the filling by combining the ricotta, 1 ½ cups of Italian 4 cheese blend, minced onion, egg, chopped parsley, salt, and pepper until well blended.
Pour 1 ½ cups of prepared marinara sauce into the bottom of a 9x13-inch baking dish.
Fill each giant shell with about one tablespoon of filling and place them over the sauce.
Spoon the rest of the sauce over the stuffed shells and sprinkle the remaining ½ cup of Italian 4 cheese blend evenly over them.
Bake the shells for 30 minutes and serve with chopped basil on top.