Place a large pot, filled with water, over high heat and bring to a boil. Add a small handful of salt followed by the pasta. Bring to a boil and stir. Boil pasta for 7 to 8 minutes or until al dente. Drain pasta into a colander. Set aside.
Place a large skillet over medium heat and add 1 1/2 tablespoons oil. Once hot, add sausage. Using a wooden spoon, break sausage apart until evenly crumbled and browned.
Transfer sausage to a plate and return skillet to stove.
Add remaining oil. Once hot, add shallot and fennel (if using) and sauté for 2 to 3 minutes. Season with salt and pepper.
Stir in garlic and continue to sauté for 30 seconds.
Add tomato paste and stir for 1 minute.
Deglaze the skillet with the wine or stock, scraping up the bits on the bottom with a wooden spoon. Continue to cook until the liquid has mostly evaporated.
Stir in cream and reduce heat to medium-low. Season with salt and pepper and stir. Simmer sauce until reduced by half, 3 to 4 minutes.
Stir sausage back into skillet, along with kale and pepper flakes and simmer for 3 to 4 minutes.
Gently stir in Parmesan until fully incorporated and sauce is smooth and creamy.
Add pasta to skillet and gently stir into sauce. Season with salt and pepper and serve with more grated Parmesan and crushed red pepper flakes.