The secret to bright green broccoli rabe, without a hint of bitterness, is pre-cooking it in boiling water and cooling for up to 1/2 hour in an ice-water bath.
Trim and chop broccoli rabe into 2-inch pieces. In a large pot bring water and salt to a boil. Drop broccoli rabe in boiling water and cook for 3 minutes. Drain and cool in ice water for 1/2 to 1 hour. Drain well. In a large skillet heat the olive oil over medium-high heat. Add garlic and red pepper flakes and cook until the garlic is golden brown. Add broccoli rabe, cover, and lower heat to medium-low. Let cook for 4 minutes to heat broccoli rabe. Toss with salt to taste just before serving.