01 - Preheat oven to 350°F (180°C).
02 - Cook jumbo pasta shells in salted boiling water until al dente. Drain and arrange on a kitchen towel to cool evenly.
03 - In a skillet, heat olive oil over medium heat. Sauté the chopped onion and minced garlic until softened. Stir in tomato passata or marinara sauce, red pepper flakes if using, and season with salt and pepper. Simmer for 10 minutes, then remove from heat.
04 - In a mixing bowl, combine ricotta cheese, chopped spinach, grated mozzarella, Parmesan, dried cranberries, egg, ground nutmeg, minced garlic, salt, and pepper. Mix thoroughly until well incorporated.
05 - Spread half the sauce evenly on the bottom of a large ovenproof baking dish.
06 - Using a spoon, carefully fill each cooked pasta shell with the ricotta mixture. Arrange stuffed shells closely together in the prepared baking dish.
07 - Pour the remaining tomato sauce evenly over the filled shells. Generously sprinkle extra grated Parmesan on top.
08 - Cover the baking dish with aluminum foil and bake for 25 minutes. Remove foil and continue baking for 10 minutes until the top is golden and bubbling.
09 - Garnish with fresh basil leaves before serving hot.