01 - Preheat the oven to 350°F.
02 - Boil jumbo conchiglie in a large pot of salted water until just al dente, approximately 2 minutes less than package directions. Drain and transfer to a towel to cool.
03 - Wilt fresh spinach in a dry skillet for 2 to 3 minutes. Let cool, then squeeze out excess moisture and chop finely.
04 - In a mixing bowl, blend chopped spinach, ricotta, Parmesan, egg, minced garlic, ground nutmeg, salt, and black pepper until thoroughly combined.
05 - Heat olive oil in a saucepan over medium heat. Sauté onion until softened, about 5 minutes. Add garlic and cook for 1 minute. Stir in passata, oregano, sugar, salt, and pepper. Simmer for 10 minutes, stirring occasionally.
06 - Spread half of the tomato sauce evenly across the bottom of a large baking dish. Fill each pasta shell with the spinach-ricotta mixture using a spoon or piping bag, and arrange them in the dish.
07 - Pour the remaining tomato sauce over the arranged shells. Sprinkle mozzarella and additional Parmesan cheese on top.
08 - Cover the baking dish loosely with foil and bake at 350°F for 25 minutes.
09 - Remove foil and bake for an additional 10 minutes, until the dish is bubbly and golden on top.
10 - Optionally garnish with fresh basil leaves before serving.