Use a sharp knife to cut the thick bottom part from the stem, about the bottom two inches. Cut the remaining stalks, leaves and florets into slices about 1 inch across. You can chop it into smaller pieces if you like, which is nice if your broccoli rabe is large and mature, with bigger florets and leaves and thicker stems.
Heat the oil in a large pan with a lid. Add the onion, season with salt and pepper halved and sliced and saute for about 8 minutes until the onion is softened and browned. Then add the broccoli rabe and saute for 3 minutes.
Stir in the red pepper flakes, add 1/3 cup of water (or broth) to the pan, stir, cover the pan, and allow the broccoli rabe to cook for about 5 more minutes until it is done to your liking. Adjust seasonings as needed.