Preheat the oven to 350 degrees F. Lightly grease a 9x13 inch baking dish or dish of similar size.Heat the olive oil in a large pot over high heat. When the oil shimmers, add the chicken sausage and brown all over, about 5 to 8 minutes. Reduce the heat to low, add the bell pepper, pumpkin, vodka, milk, oregano, crushed red pepper flakes, and a pinch each of salt and pepper. Simmer for 15 minutes or until the sauce thickens slightly. Taste and adjust seasonings as needed.Meanwhile, bring a large pot of salted water to a boil over high heat and boil the pasta until al dente according to package directions.In a medium bowl, combine the ricotta, fontina, and 1/2 cup pesto. To assemble, spoon 3/4 of the pumpkin sauce into the bottom of the prepared baking dish. Working with one pasta shell at a time, spoon about 1 tablespoon of the cheese mixture into each shell, placing the filled shell into the baking dish as you go. Spoon the remaining pumpkin sauce over top of the shells and then drizzle the remaining 1/4 cup pesto evenly over everything. Add the mozzarella. Transfer the baking dish to the oven and bake 20-25 minutes or until the cheese has melted and is lightly browned on top. Cool 5 minutes before serving. Serve with fresh basil. Enjoy!