Bring a pot of salted water to a boil. Cook pasta until al dente, about 10 minutes or as package label directs. Add broccoli rabe for the last 2 minutes of cooking time.
While pasta is cooking, warm olive oil in a large skillet over medium heat. Add sausage and cook, breaking it up into small pieces with a wooden spoon, until brown and cooked through, 8 to 10 minutes. Add garlic and cook, stirring, until fragrant, about 2 minutes.
Reserve 1 cup cooking liquid; drain pasta and broccoli rabe, add to skillet with 1/2 cup reserved water and toss well. Sprinkle Parmesan on top and toss to combine, adding more liquid if necessary to moisten. Season with salt and pepper. Serve, passing additional Parmesan, if desired.