Preheat Oven: Preheat your oven to 375°F (190°C).
Cook Pasta Shells: Cook the jumbo pasta shells according to the package instructions until they are al dente. Drain and rinse under cold water to stop the cooking process. Set aside.
Prepare Filling: In a mixing bowl, combine the ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, egg, chopped parsley, salt, pepper, and garlic powder. Mix well. If using any optional additions like spinach, mushrooms, or meat, fold them into the mixture now.
Fill the Shells: Spread 1 cup of marinara sauce in the bottom of the baking dish. Fill each pasta shell with the cheese mixture using a spoon or a piping bag. Arrange the filled shells in the baking dish.
Bake: Spoon the remaining marinara sauce over the shells, and sprinkle with additional mozzarella cheese. Cover the dish with aluminum foil and bake for 25 minutes. Then, remove the foil and bake for an additional 5 minutes, or until the cheese is melted and slightly golden.
Serve: Let the dish cool for a few minutes before serving. Garnish with fresh parsley if desired.