1. Preheat your oven to 375°F (190°C) and lightly grease a 9×13-inch baking dish.
2. Bring a large pot of salted water to a boil. Add the jumbo pasta shells and cook until al dente, about 8-10 minutes. Drain and rinse under cold water to stop cooking, then set aside on a clean towel to dry.
3. While the pasta cooks, heat olive oil in a skillet over medium heat. Add minced garlic and sauté for about 1 minute until fragrant but not browned.
4. Add chopped spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat and let cool slightly.
5. In a medium bowl, combine ricotta, half of the mozzarella, Parmesan, egg, cooked spinach and garlic, a pinch of nutmeg (if using), salt, and pepper. Mix gently until well incorporated.
6. Spread a thin layer of marinara sauce on the bottom of your baking dish to prevent sticking.
7. Using a spoon, fill each cooked shell generously with the ricotta-spinach mixture and place them in the baking dish, open side up.
8. Pour the remaining marinara sauce evenly over the stuffed shells, then sprinkle the remaining mozzarella and extra Parmesan on top.
9. Cover the dish loosely with foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden.
10. Let the shells rest for 5 minutes before serving to allow the filling to set slightly.

From my experience, letting the shells rest after baking makes all the difference—it helps everything settle so you get perfect bites every time.