Cook the spaghetti according to package instructions until al dente. Reserve 1/2 cup of pasta water and drain the rest.

In a large skillet, heat olive oil over medium-high heat. Add fennel and cook, stirring occasionally, until softened and caramelized, about 10 minutes.

Add the garlic and fennel seeds to the skillet, and sauté for an additional 1-2 minutes until fragrant.

Push the fennel mixture to one side of the skillet. Add the sausage to the cleared side and cook, breaking it apart with a spoon, until browned and cooked through.

Toss the cooked pasta into the skillet, adding reserved pasta water as needed to create a sauce. Stir in Parmesan, parsley, and season with salt and pepper.