01 - Preheat the oven to 350°F.
02 - Cook the pasta shells in salted boiling water until al dente; drain and set aside on a kitchen towel to cool.
03 - In a skillet, heat olive oil over medium heat. Sauté onion and garlic until softened. Add passata or marinara, red pepper flakes, salt, and pepper. Simmer for 10 minutes.
04 - In a mixing bowl, combine ricotta, spinach, mozzarella, Parmesan, chopped cranberries, egg, nutmeg, minced garlic, salt, and pepper. Mix well.
05 - Spread half the tomato sauce on the bottom of a large ovenproof baking dish.
06 - Fill each pasta shell with the ricotta mixture using a spoon. Arrange the filled shells in the dish.
07 - Pour remaining sauce over the shells and sprinkle extra Parmesan on top.
08 - Cover with foil and bake for 25 minutes.
09 - Remove foil, bake another 10 minutes until golden and bubbling.
10 - Garnish with fresh basil before serving.