Heat the olive oil in a large heavy pot or Dutch oven over medium heat. Add the fennel and onion and saute for 7 minutes, stirring occasionally, until both the onion and fennel soften and turn shiny and translucent. You want to keep an eye, though, because you don't want too much caramelization. A little brown is ok, though! That will add a lovely layer of sweetness underneath the spice.
Add the sausage and use a fork or flat wooden spoon to break it up. Cook for about 8 minutes, until nicely browned.
If you haven't toasted your fennel seeds yet, now is the perfect time. I like to dry roast them in a pan. Which means roasting them over low to medium low heat in a dry skillet....no oil, butter or anything except the fennel seeds. This should only take 2-3 minutes -- once you smell the fennel, they're done! And, YES, you need to take a couple minutes to toast your fennel seeds -- it adds even more beautiful fennel flavor to the dish!
Add the garlic, crushed fennel seeds, red pepper flakes, 2 teaspoons salt, and 1 teaspoon black pepper and cook for one minute.
Pour in the wine, bring to a boil, and add the heavy cream, half-and-half, and tomato paste.
Bring the sauce back to a boil, lower the heat, and simmer for about 20-30 minutes, until the sauce has thickened.
Bring a large pot of water to a boil, add 2 tablespoons salt, and cook the rigatoni. To make sure the rigatoni is al dente and not mushy, cook it for 3 minutes less than the package recommends. Drain and add the rigatoni directly to the sauce. Stir to coat the pasta really well with the sauce. Cook over very low heat for 5 minutes. This will help the sauce coat the pasta.
Take the pan off the heat, stir in the parsley and 1/2 cup of the Parmesan.
Serve hot in shallow bowls, sprinkle each serving with the remaining Parmesan, extra parsley, a sprinkle of Maldon salt and drizzle of olive oil.
Serve immediately.