1. Bring a large pot of salted water to a boil and cook the jumbo pasta shells until al dente according to the package directions. Drain and rinse gently with cool water.

2. Preheat the oven to 375°F. Spread 1 cup of marinara sauce across the bottom of a 9×13-inch baking dish.

3. In a large bowl, mix the ricotta cheese, 1 cup mozzarella cheese, 1/2 cup Parmesan cheese, egg, chopped spinach, minced garlic, Italian seasoning, salt, and black pepper until well combined.

4. Spoon the ricotta mixture into each cooked shell and place the filled shells in the prepared baking dish.

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

6. Cover loosely with foil and bake for 25 minutes. Remove the foil and bake for 10 to 15 minutes more, until the cheese is melted and bubbly.

7. Let the dish rest for 5 to 10 minutes, then garnish with fresh parsley and serve.