In the South, hardy greens are often cooked with some kind of pork, but there is no reason vegetarians can’t enjoy the pleasantly bitter, robustly flavored heap of greens that are part and parcel of many Southern meals.
Heat the oil in a large, deep skillet over medium-high heat. Add the onions and stir for about 3 minutes until they start to turn golden. Add the garlic and stir for another 2 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 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, stir to combine, and serve hot, spooning up some of the liquid (pot likker) to serve with the greens.
Notes
To wash mustard greens, fill a very large bowl or a sparkling clean sink with cold water. Submerge the greens and swish them around with your hands vigorously. Lift out the greens, then pour out or drain the water. Rinse the sink or the bowl well, and repeat 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 finished.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 greens, you’ll want to cut them out, as they can be tough. Hold the stem and kind of fold the leaf in half over the stem. Then tear the stem up and away from the leaf. You can also slice the stem from the leaf if you find that easier.
Storage
You can store these in a tightly covered container for up to 5 days in the fridge. Reheat gently over medium-low heat until hot throughout, stirring frequently.Cooked mustard greens can also be frozen with their liquid. Place them, along with some of the cooking liquid, also known as pot likker, in freezer-proof zipper-top bags. Press out any additional air and freeze for up to 6 months. Thaw overnight in the refrigerator or on the counter for a few hours before reheating.