Bring a large pot of salted water to a boil and cook pasta according to package directions until al dente. Reserve 1 cup of pasta cooking water, then drain and set aside.
Heat olive oil in a large skillet over medium-high heat. Add crumbled spicy Italian sausage and cook 6–8 minutes until browned. Transfer to a plate, leaving some fat in the pan.
Reduce heat to medium. Add chopped onion to the skillet and cook until translucent, about 4 minutes. Add garlic and red pepper flakes and cook 30 seconds until fragrant.
Stir in tomato paste, then add crushed tomatoes. Simmer 5–7 minutes to thicken. Stir in heavy cream and return sausage to the pan. Simmer 2–3 minutes and season with salt and pepper.
Add drained pasta to the skillet and toss to coat. Add reserved pasta water a little at a time if the sauce is too thick. Stir in Parmesan until glossy. Serve garnished with basil and extra Parmesan.
Notes
Reduce red pepper flakes for less heat, or use mild sausage and add smoked paprika for depth.
Swap heavy cream for Greek yogurt for a lighter finish (stir in off heat).
Sauce freezes well; cook fresh pasta when reheating for best texture.