Preheat the oven to 375°F. Lightly coat a 9 x 13-inch baking dish with nonstick cooking spray. Pour 1½ cups of the marinara sauce into the dish and spread it evenly over the base.
In a large mixing bowl, add the ricotta, 1½ cups shredded mozzarella, Parmesan, egg, Italian seasoning, salt, and black pepper. Stir thoroughly until the ingredients are evenly incorporated and the filling has a smooth, uniform consistency.
Fill each cooked jumbo pasta shell with the prepared ricotta mixture. Arrange the stuffed shells snugly in a single layer over the marinara sauce in the baking dish.
Spoon the remaining 1½ cups of marinara sauce evenly over the stuffed shells, ensuring the shells are covered. Distribute the remaining 1½ cups of shredded mozzarella uniformly across the top.
Cover the baking dish with aluminum foil and bake for 20 minutes. Remove the foil, then return the dish to the oven for another 10 minutes, allowing the cheese to melt completely and develop a light golden color.
Remove the stuffed shells from the oven and scatter the freshly chopped parsley over the surface. Serve while hot.