Heat the olive oil in a large heavy pot or Dutch oven over medium heat. Add the fennel and onion and sauté for about 7 minutes, stirring occasionally, until tender.

Add the sausage and cook for 7–8 minutes, breaking it apart with a fork, until nicely browned.

Add the garlic, crushed fennel seeds, red pepper flakes, 2 teaspoons salt, and 1 teaspoon black pepper. Cook for 1 minute, stirring frequently.

Pour in the white wine or chicken broth and bring to a boil. Add the heavy cream, half-and-half, and tomato paste. Bring the sauce back to a boil, then reduce the heat and simmer for about 20 minutes, until the sauce has thickened.

Meanwhile, bring a large pot of salted water to a boil. Cook the pasta according to the package directions until al dente. Reserve about ½ cup of the pasta water, then drain the pasta.

Add the drained pasta to the sauce and stir well to coat. Cook over low heat for about 5 minutes, allowing the pasta to absorb some of the sauce. If the sauce becomes too thick, add a little of the reserved pasta water.

Remove from the heat and stir in the parsley and ½ cup of the Parmesan.

Serve hot in shallow bowls with the remaining Parmesan sprinkled over the top or served on the side.