Heat oil in a large skillet over medium heat. Add sausage, and cook, turning occasionally until browned, about 10 minutes. Remove sausage to a plate and reserve pan and sausage drippings.
Meanwhile, bring a large pot of cold water to a boil over high heat. Add a large pinch of salt and broccoli rabe and cook, uncovered, until broccoli rabe is bright green and tender but still slightly crisp, about 5 minutes. Immediately transfer broccoli rabe using tongs to a colander (reserve boiling water) and rinse under cold running water. Chop into bite-sized pieces and set aside.
Add pasta to reserved boiling water and cook, stirring occasionally, until al dente (tender but not mushy), about 12 minutes. Meanwhile, heat reserved pan and drippings over medium heat. Add garlic and cook, stirring frequently, until it is golden brown, about 5 minutes. Add broccoli rabe, pepper flakes, and 1 1/2 teaspoons of salt; increase heat to medium-high, and cook, continually stirring, until broccoli rabe is hot and tender, about 5 minutes. Slice sausage into 1/4-inch thick coins and add to pan and mix well cook until sausage is warmed through, about 5 more minutes.
Drain pasta in a colander in sink, reserving 1/4 cup of cooking liquid. Toss pasta and liquid with broccoli rabe mixture. Transfer pasta to a serving dish or individual bowls, garnish with Parmesean and serve.
