Heat 2 tablespoons of oil in a large lidded saucepan on medium heat. Add onion and garlic and let simmer for 1-2 minutes. Add sausage to the skillet. Use the end of a spatula to break the sausage into small chunks, cooking until browned and fully cooked through, about 5-7 minutes. Season with salt, pepper, and crushed red pepper flakes. Discard excess fat.
Add marinara, wine, and basil. Cover and let simmer for 5-7 minutes.
Meanwhile, bring a large pot of salted water to a boil. Cook shells al dente.
Strain shells and add to the sauce along with the cream. Let simmer for an additional 5 minutes and serve with fresh basil and Parmesan cheese.
Notes
This can be made dairy free by using homemade cashew cream in place of cream. It's just as creamy and delicious.