1. Preheat the oven to 375°F (190°C).

2. Bring a large pot of salted water to a boil. Cook the jumbo pasta shells until just al dente. Drain and set aside.

3. In a skillet, heat olive oil over medium heat. Add onion and sauté until softened, about 4 minutes.

4. Add garlic and cook for 30 seconds.

5. Stir in ground turkey, season with salt, pepper, Italian seasoning, and red pepper flakes. Cook until browned and fully cooked.

6. Add spinach and cook for another 2–3 minutes, until wilted. Remove from heat and let it cool.

7. In a large bowl, mix ricotta, half of the mozzarella, half of the Parmesan, and the egg.

8. Add the cooled turkey-spinach mixture to the cheese mixture and stir until combined.

9. Spread 1 cup of marinara sauce on the bottom of a baking dish.

10. Fill each pasta shell with the meat and cheese filling and place in the dish.

11. Spoon remaining marinara over the shells. Top with the remaining mozzarella and Parmesan.

12. Cover with foil and bake for 25 minutes.

13. Remove foil and bake an additional 10 minutes, until bubbly and golden.

14. Let rest for 5–10 minutes before serving. Garnish with chopped parsley or basil, if desired.