01 - Set oven temperature to 350°F (180°C) and preheat.
02 - Boil jumbo pasta shells in a large pot of salted water until al dente. Drain and reserve.
03 - Heat olive oil in a saucepan over medium heat. Sauté chopped onion for about 3 minutes until softened. Add minced garlic and cook for 1 additional minute. Stir in tomato passata, oregano, basil, salt, and pepper. Simmer 10 to 15 minutes.
04 - Wilt fresh spinach in a hot pan, then chop finely or, if using frozen, squeeze out excess moisture. In a large bowl, thoroughly combine ricotta, prepared spinach, Parmesan, egg, minced garlic, nutmeg, salt, and black pepper until smooth.
05 - Spread half of the tomato sauce evenly on the bottom of a 9 x 13 inch baking dish.
06 - Fill each cooked shell with the ricotta-spinach mixture and place, open side up, into the baking dish.
07 - Spoon the remaining tomato sauce over stuffed shells. Sprinkle with mozzarella and additional Parmesan cheese.
08 - Cover the dish with foil and bake for 20 minutes.
09 - Remove foil and bake an additional 10 to 15 minutes, until the surface is bubbling and golden.
10 - Let cool for 5 minutes before serving.