Heat the olive oil in a large Dutch oven or deep sauté pan over medium-high heat. Add the sausage and cook, breaking it into bite-sized pieces, until deeply browned and cooked through, about 8 to 10 minutes. Transfer the sausage to a plate and set aside.

Reduce the heat to medium and add the fennel and onion to the pot. Season with a pinch of salt and pepper and cook, stirring occasionally, until softened and lightly caramelized, about 10 minutes.

Stir in the garlic, crushed fennel seeds, and red pepper flakes. Cook until fragrant, about 1 minute.

Push the vegetables to one side of the pot. Add the tomato paste and cook, stirring constantly, until it darkens slightly, about 2 minutes. Stir everything together.

Pour in the white wine, scraping up any browned bits from the bottom of the pot. Simmer until reduced by about half, 3 to 5 minutes.

Add 5 cups of the chicken broth and bring to a simmer. Add the uncooked rigatoni, stir well, and bring to a gentle boil.

Reduce the heat to maintain a steady simmer. Cook uncovered, stirring frequently, until the pasta is al dente and most of the liquid has been absorbed, about 15 minutes. If needed, add additional broth a splash at a time to keep the sauce loose.

Once the pasta is tender, stir in the heavy cream and Dijon mustard. Simmer for 1 to 2 minutes, until slightly thickened.

Add the reserved sausage, Parmesan cheese, lemon zest, and parsley. Stir until the cheese has melted and the sauce is silky and glossy. If desired, stir in 1 tablespoon of butter for extra richness.

Taste and adjust the seasoning with additional salt and pepper as needed.

Divide among bowls and serve topped with extra Parmesan, parsley, and freshly cracked black pepper.