In a large sauce pot, bring 1 gallon of salted water to a boil. Add the broccoli rabe and blanch for 2 minutes. Drain.
Meanwhile, heat a large skillet over medium-high heat. Add oil, garlic and red pepper flakes. Allow garlic to brown.
Add broccoli rabe to skillet with toasted garlic. Toss to combine. Toss in Parmesan and lemon zest. Season with salt and pepper. Cook over medium-high heat for 2 to 3 minutes.