01 - Preheat oven to 350°F.
02 - Boil jumbo pasta shells in salted water until al dente. Drain thoroughly and arrange on kitchen towels to cool.
03 - Heat olive oil in skillet over medium heat. Sauté onion and garlic until softened, about 3 minutes. Add tomato passata, red pepper flakes, salt, and pepper. Simmer for 10 minutes until slightly thickened.
04 - Combine ricotta, spinach, mozzarella, Parmesan, chopped cranberries, egg, nutmeg, minced garlic, salt, and pepper in large mixing bowl. Mix until fully incorporated.
05 - Spread half of the tomato sauce evenly across the bottom of a large baking dish.
06 - Fill each cooled pasta shell with ricotta mixture using a spoon. Arrange filled shells in single layer in prepared dish.
07 - Pour remaining tomato sauce over stuffed shells. Sprinkle additional Parmesan cheese evenly across the top.
08 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes.
09 - Remove foil and continue baking for 10 minutes until cheese is golden and sauce is bubbling around edges.
10 - Arrange fresh basil leaves over the top. Serve hot while cheese is melted and creamy.