Heat 2 tablespoons olive oil in a large (12-14-inch) skillet over medium-high heat. Crumble the sausage and add to the pan. Cook until the sausage is cooked and browned. Scoop out the sausage to a plate.
Pour the wine in the pan and let it bubble for a minute, while scraping up the brown juices. Lower the heat and add more olive oil to coat the bottom of the pan, red onion and fennel, along with 1 teaspoon salt. Cook without browning (adjust the heat if necessary), until the vegetables are tender, about 12 minutes.
Meanwhile, bring 6 quarts water to a boil and add 3 tablespoons kosher salt.
Chop the broccoli rabe bunch into halves or thirds. Add to the boiling water and cook 2 minutes. Remove with a slotted spoon to colander set into a large bowl.
Add the pasta to the water. While the pasta cooks, stir the garlic and crushed chili to taste (I start with 1/2 teaspoon) into the pan with the fennel. Scoop out about 1/4 cup of the boiling pasta water and add to the pan to loosen the sauce.
When the pasta is al dente, drain, and reserve another 1/4 cup or so of the cooking water. Add the pasta to the pan along with the broccoli rabe and stir everything together over low heat. Dribble in more water and olive oil as needed to keep it saucy (but not too watery).
Serve the pasta in bowls, sprinkled with lightly chopped fennel frond tops, and the cheese.