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With fall and winter approaching, the braising greens are entering their peak season. Relatively tender greens like Swiss chard or mustard greens can be cooked rapidly with a quick stir-fry or sauté (try cooking them with just a bit of slivered garlic and oil). Tougher greens like collards or kale require longer cook times to beat them into submission.
Either way, in most cases, you'll want to separate the stalks from the stems before you cook them. The stalks are thicker, so they take longer to cook, particularly for fast cooking recipes where the leaves wilt and cook in a matter of seconds (depending on how you like the finished texture, this matter less for longer braises).
Once you know how to separate stalk from stem, prepping greens is a simple affair.
Shopping and Storage
When shopping for greens, look for ones with bright, firm stalks with no signs of discoloration (the stalks will begin to brown around the edges before the rest of the leaves do). The leaves should be full and crisp, not wilted. In a well-maintained market, unless it's coming from from the farm, winter greens should be sold either under a mister, or under crushed ice.
Sandy or dirt-covered stems and leaves are totally normal. Just make sure to wash all greens carefully under cool running water and dry them on paper towels or a clean kitchen towel before using.
Winter greens seem fairly hearty, but they wilt quite easily. The best way to store them is to wrap them in a moist paper towel and place them in a plastic vegetable bag or a zipper-lock bag left slightly ajar in the vegetable crisper. Stored this way, greens should keep for at least 3 to 5 days, often a week or longer.