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