1. Preheat the oven to 375°F and lightly grease a 9×13-inch baking dish with olive oil.

2. Bring a large pot of salted water to a boil and cook the jumbo shells until just al dente. Drain and lay them out so they do not stick together.

3. In a large mixing bowl, combine the ricotta cheese, 1 cup mozzarella, 1/2 cup Parmesan, egg, parsley, garlic, Italian seasoning, salt, and black pepper. Stir until smooth and fully combined.

4. Spread about 1 cup of marinara sauce across the bottom of the prepared baking dish.

5. Fill each cooked shell with the ricotta mixture and arrange them open side up in the dish.

6. Spoon the remaining marinara sauce over the shells and sprinkle the top with the remaining 1 cup mozzarella and 1/4 cup Parmesan.

7. Cover the dish with foil and bake for 25 minutes.

8. Remove the foil and bake for 10 more minutes, or until the cheese is melted and the sauce is bubbling.

9. Let the shells rest for 5 minutes before serving.