01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Boil jumbo pasta shells in salted water until al dente. Drain and let cool slightly.
03 - In a bowl, mix ricotta, mozzarella, Parmesan, egg, garlic, nutmeg, salt, pepper, and spinach until well combined.
04 - Spread 1 cup marinara sauce evenly in the bottom of the baking dish.
05 - Fill each pasta shell with about 2 tablespoons of filling and arrange them side by side, open side up, in the dish.
06 - Pour remaining marinara sauce over the stuffed shells. Sprinkle with mozzarella and Parmesan cheese.
07 - Cover with foil and bake for 25 minutes.
08 - Remove foil and bake uncovered for 10 minutes until cheese is bubbly and golden.
09 - Let stand for 5 minutes, then garnish with chopped basil or parsley before serving.