mustard greens

The peppery leaves of the mustard plant are a popular soul food ingredient, ranking second only to collard greens. The leaves are a rich, dark green and have a pungent mustard flavor. Though they can be found year-round in some locales, fresh mustard greens are most abundant from December through early March. They're also available frozen and canned. When choosing fresh greens, look for crisp young leaves with a rich green color. Reject those with yellow, flabby or pitted leaves or thick, fibrous stems. Refrigerate greens, tightly sealed in a plastic bag, for up to a week. Wash them just before using. Mustard greens can be steamed, sautéed or simmered. They are usually served as a side dish, often flavored with onion, garlic, ham, salt pork or bacon. Mustard greens, a cruciferous vegetable, are an excellent source of vitamins A and C, thiamine and riboflavin. See also mustard; mustard oil; mustard; prepared.

The noun has one meaning:

Meaning #1: leaves eaten as cooked greens
  Synonyms: mustard, leaf mustard, Indian mustard