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

2. Cook the jumbo pasta shells in salted boiling water until just shy of al dente, then drain and lay them out so they do not stick together.

3. Heat olive oil in a skillet, add garlic, and cook until fragrant. Add spinach and cook until wilted, then squeeze out excess moisture.

4. In a bowl, mix ricotta, 1 cup mozzarella, 1/2 cup Parmesan, egg, spinach, Italian seasoning, nutmeg, salt, and black pepper until creamy.

5. Spread 1 cup marinara sauce in the bottom of the baking dish.

6. Fill each pasta shell with the spinach ricotta mixture and arrange the shells open side up in the dish.

7. Spoon the remaining marinara sauce around and lightly over the shells, then top with remaining mozzarella, remaining Parmesan, and red pepper flakes.

8. Cover with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes, until the cheese is melted and the sauce is bubbling.

9. Let the pasta rest for 5 minutes, then garnish with fresh parsley and serve warm.