01 - Preheat oven to 350°F (180°C).
02 - Boil jumbo pasta shells in salted water until al dente. Drain and set shells on a kitchen towel to cool.
03 - In a skillet, heat olive oil over medium heat. Sauté chopped onion and minced garlic until softened. Add tomato passata or marinara sauce, red pepper flakes, salt, and black pepper. Simmer for 10 minutes.
04 - In a large mixing bowl, combine ricotta, chopped spinach, mozzarella, Parmesan, chopped dried cranberries, egg, ground nutmeg, minced garlic, salt, and black pepper. Stir until well blended.
05 - Spread half of the tomato sauce on the bottom of a large ovenproof baking dish.
06 - Fill each cooled pasta shell with the ricotta mixture using a spoon. Arrange filled shells in the prepared baking dish.
07 - Pour the remaining tomato sauce evenly over the stuffed shells. Sprinkle extra grated Parmesan cheese on top.
08 - Cover the dish with aluminum foil and bake for 25 minutes.
09 - Remove foil and continue baking for an additional 10 minutes until the top is golden and bubbling.
10 - Garnish with fresh basil leaves just before serving.