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 sauté for about 8 minutes until the onion is softened and browned. Add the garlic and cook for another minute, stirring occasionally. Then add the broccoli rabe and sauté for 3 minutes.
Stir in the red pepper flakes, add ⅓ 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.
Notes
Remember that the broccoli rabe will continue to cook and soften a bit after you remove the vegetable from the heat, even after you turn it out of the pan into a serving dish.
You can add other ingredients to this dish to change up the flavor. Try sliced or halved olives, a sprinkle of feta or parmesan cheese, some fresh herbs, like oregano, or minced red bell pepper. You may also choose to sprinkle in some balsamic vinegar at the end, which is a really nice counterpoint to the bitterness of the vegetable. A squeeze of fresh lemon at the end is also a nice finishing touch.