Fill a large pot about ⅔ full with salted water, enough to cover the rapini when you add it later. Place the pot on the stove with high heat and cover it.
Rinse the greens well in cold water to remove any dirt. Cut off the bottom four inches of broccoli rabe stems. If you still have some stems that are thick, you can remove the leaves from them to use and then discard the thick stems. See notes for an easy way to remove the leaves as in Naples.
Peel the garlic and slice it thinly, then set it aside.
When the water is boiling, add the rapini to the pot. Wait for the water to come to a boil again, and when it does, leave the greens in for one minute longer. Drain the broccoli rabe and let it sit in the drainer in the sink.
Heat olive oil in a large skillet. Add the garlic and let it cook for a minute. Pat the rapini dry with a paper towel. Add the greens to the pan, turning them to coat with olive oil. Add salt and black pepper to taste. Cook for 5 minutes or until done to your liking (stirring occasionally). Taste again for seasonings, and add red pepper flakes before serving.
If you wish to remove the leaves from any thick stems so you can keep the leaves and discard the thick stems (as this dish is prepared in Naples,) you can do this one stem at a time:
Make a fist around the bottom of a stem with your right hand.
With your left hand, make a fist around the stem just above your right hand.
Slide your left fist along the length of the stem (sliding to the left). Most of the leaves will pull off.