Line a plate with paper towels. Heat the oil in a large pot over medium-low heat. Add the bacon and sauté until crisp, about 4 minutes. Remove the bacon with a slotted spoon to the paper towels. Leave the fat in the pan.
Add the onions and stir for about 3 minutes until they start to turn golden. Add the garlic and stir for another two minutes, until the garlic turns golden. Add the greens to the pot and stir to coat them with the oil and onions. Cook, stirring, occasionally until they begin to wilt, about 5 minutes. Pour in the broth. Add the red pepper flakes, season with salt and pepper, then turn the heat to medium-high and bring the broth to a simmer. When the broth is simmering, lower the heat to medium and cover the pot. Cook, lifting the lid and stirring occasionally, for about 20 minutes, or until the collard greens are fairly tender but not super soft. (Or cook for more or less time! Your call!)
Sprinkle the greens with the vinegar and crisped bacon, stir to combine, and serve hot, spooning up some of the pot likker to serve with the greens.
Notes
Cleaning Collard GreensFill a very large bowl or a sparkling clean sink with cold water, and dunk those greens. Swish them around with your hands vigorously, then remove them, pour out or drain the water, rinse the sink or the bowl, and repeat. Do this as many times as necessary to remove all of the dirt from the greens. The water should be totally clear with no grit on the bottom of the bowl or sink when you are done.Removing the stems is optional, but they can be pretty tough even after cooking for a while. There is a lot of nutrition in the stems, which is why some people tend to leave them in, but if you are looking for more consistently tender cooked collards, you’ll want to remove the stems. The easiest way to remove the stems is simply to hold the stem and kind of fold the leaf in half over the stem. Then, tear the stem up and away from the leaf. No worries if the leaf tears, too — you’re going to be slicing and cooking it, so that won’t matter at all.