Boil a pot of water and cook the pasta according to package instructions. Omit the salt.
Heat the olive oil in a skillet over medium heat and cook the sausage until lightly browned.
Add the fennel seed and minced garlic to the skillet and stir for 1 minute, until the garlic becomes fragrant (you can use a mortar and pestle to grind the fennel seed if you want to -- I think it goes well with the sausage.) If you don't have any fennel seed, some black pepper or red pepper flakes would also be nice.
About 4 minutes before the pasta is done cooking, add the broccoli rabe to the pasta pot and let it boil for 4 minutes until soft. Use a slotted spoon to transfer the broccoli rabe to the skillet, and mix it with the sausages.
Drain the pasta, and stir the pasta together with the broccoli rabe and sausage mixture.
You can sprinkle some Parmesan cheese on top just before serving. I preferred not to, because the sausage is already pretty salty. Sausage + Parmesan cheese is TOO salty for my taste.