Heat oil in 12-inch nonstick skillet over medium-high heat until just smoking.
Add sausage and cook, breaking up meat into rough ½-inch pieces with wooden spoon, until lightly browned, about 5 minutes.
Stir in garlic and pepper flakes and cook until fragrant, about 30 seconds; set aside.
Meanwhile, bring 4 quarts water to boil in large pot.
Add broccoli rabe and 1 tablespoon salt and cook, stirring often, until crisp-tender, about 2 minutes.
Using slotted spoon, transfer broccoli rabe to skillet with sausage mixture.
Return water to boil, add pasta, and cook, stirring often, until al dente.
Reserve 1 cup cooking water, then drain pasta and return it to pot.
Add sausage–broccoli rabe mixture, Pecorino, and ⅓ cup reserved cooking water and toss to combine.
Adjust consistency with remaining ⅔ cup reserved cooking water as needed.
Season with salt and pepper to taste.
Serve.
