01 - Preheat oven to 350°F.
02 - Boil jumbo pasta shells in salted water until al dente. Drain and arrange on kitchen towel to cool.
03 - Heat olive oil in skillet over medium heat. Sauté onion and garlic until softened. Add passata, red pepper flakes, salt, and pepper. Simmer 10 minutes.
04 - Combine ricotta, spinach, mozzarella, Parmesan, cranberries, egg, nutmeg, garlic, salt, and pepper in mixing bowl. Mix thoroughly.
05 - Spread half the tomato sauce evenly across bottom of large baking dish.
06 - Fill each shell with ricotta mixture using spoon. Arrange filled shells in single layer in dish.
07 - Pour remaining sauce over shells. Sprinkle additional Parmesan on top.
08 - Cover with foil and bake 25 minutes.
09 - Remove foil and bake 10 minutes longer until golden and bubbling.
10 - Top with fresh basil leaves before serving.