The dark, vibrant green leaves of collard greens are packed with vitamins, minerals and soluble fiber. They're also extremely satisfying to cook with, and lend themselves well to olive oil braising, soups, and slow cooking. A hardy green with a mildly cabbagy flavor, collards stand up well to longer cooking times, which softens the texture and mellows the flavor. They pair deliciously with assertive flavors, such as smokiness or meatiness (pancetta, bacon, chorizo, kielbasa, any cured meat), sweetly pungent aromatics (garlic, onions, fresh ginger,) an acid or anything spicy (vinegar, lemon juice, hot sauce, red pepper flakes, chile or curry paste, minced hot peppers) or anything creamy (think heavy cream, sour cream, and goat cheese.)
Kale, turnip greens and mustard greens are also hardy, leafy vegetables with a similar flavor profile, and they would work well in most recipes that call for collards.
Look for stiff, dark-green leaves with no yellowing or drooping and bunches with most of the stems trimmed.
Cook them very, very fast or very, very slowly. Collard leaves are quite tough and must be braised or stewed to be tender, unless you slice them ultrathin and sauté them quickly.