Meanwhile, in a large, heavy skillet, heat the oil over a medium flame. Add the sausage and cook, breaking up with a spoon, until the sausage is brown and juices form, about 8 minutes. Add the garlic and red pepper flakes, and sauté until fragrant, about 30 seconds. Add the broccoli rabe and toss to coat. Add the pasta and enough reserved cooking liquid, ¼ cup at a time, to moisten. Stir the Parmesan cheese, salt to taste, and pepper into the pasta mixture. Transfer to pasta bowls and serve.