Bring 1-inch of water to a boil in a deep skillet. Add rabe, season with salt, and cover pan. Reduce heat to simmer and cook 10 minutes. Drain rabe.
Return pan to heat and saute garlic and red pepper flakes in oil over moderate heat for 1 to 2 minutes. Add rabe, coat in garlic oil. Cook rabe in garlic oil for 2 minutes and remove from heat.
Squeeze the juice of lemon over the pan and sprinkle in zest. Toss rabe to distribute lemon flavor and transfer broccoli rabe to serving dish.