Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add the ground turkey or beef to the skillet, breaking it apart with a spoon. Cook until browned, about 7-10 minutes.
Sprinkle in the red pepper flakes and Italian seasoning, mixing well. Pour in the crushed tomatoes and add sugar, salt, and pepper. Simmer for 10 minutes to allow flavors to meld.
While the sauce simmers, prepare the whipped ricotta. In a medium bowl, combine ricotta cheese, heavy cream, and Parmesan cheese. Use a hand mixer or whisk to whip until smooth and creamy.
Preheat the oven to 375°F.
In a large baking dish, spread a thin layer of the meat sauce on the bottom. Fill each cooked shell with a spoonful of the whipped ricotta and place them in the dish.
Pour the remaining meat sauce over the shells and sprinkle with shredded mozzarella cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil before serving.