01 - Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.
02 - Cook jumbo pasta shells in a large pot of salted boiling water according to package directions until just al dente. Drain gently, then cool slightly in a colander.
03 - In a mixing bowl, combine pumpkin purée, ricotta, Parmesan, mozzarella, basil pesto, chopped red chili or red pepper flakes, smoked paprika, ground nutmeg, salt, and black pepper. Stir thoroughly until the mixture is creamy and homogenous.
04 - Spread 1 cup of marinara sauce evenly over the base of the prepared baking dish.
05 - Fill each cooled pasta shell with approximately 2 tablespoons of the pumpkin pesto mixture. Arrange shells open side up in the baking dish.
06 - Pour the remaining marinara sauce over the filled shells. Sprinkle evenly with shredded mozzarella and grated Parmesan cheese.
07 - Cover the dish with aluminum foil and bake for 25 minutes. Uncover and bake an additional 10 minutes, until the top is golden and bubbling.
08 - Remove from the oven and garnish with torn fresh basil leaves before serving hot.