Cook rigatoni according to package directions in well-salted water. Reserve 2 cups of the starchy water when you drain.

Heat olive oil in a large, deep pot over medium-high heat. Add the chopped onion. Sauté until softened, about 5-7 minutes. Add the Italian sausage. Cook, breaking into small pieces, until browned all the way through.

Turn the heat down. Add the minced garlic and tomato paste. Sauté until fragrant, about 1-2 minutes. Stir in the heavy cream. Bring to a low simmer. Stir in the kale and cook until wilted, about 3-5 minutes. Season with salt and red pepper flakes.

Toss the cooked pasta with the sauce. Add reserved pasta water as needed to reach desired consistency. Stir in freshly grated Parmesan cheese. Serve immediately.