Mustard greens (Brassica juncea) are the leaves of the brown or Indian mustard plant, eaten for their broad, ruffled, bright-green blades and their sharp, peppery, almost horseradish-like bite. That raw heat — the same mustard-oil chemistry that powers mustard, horseradish and wasabi — mellows markedly once the leaves are cooked, which is why they are most often braised slow and long. They are a cornerstone green of three great cuisines at once: the slow-stewed pot greens of the American South, the Punjabi sarson ka saag of northern India, and the leaf mustards and pickles of southern China.