Which leafy vegetable is the main ingredient in the Southern dish mustard greens?

Mustard greens are the main ingredient in the Southern dish called mustard greens.

Mustard greens are the leaves of several plants in the mustard family, Brassica. They have a naturally peppery flavor that becomes milder when cooked. Southern preparations often simmer them with onion, vinegar, hot pepper, and smoked or salted meat.

They are related to, but not identical to, collard greens and turnip greens. All three appear in Southern and soul food cooking, and recipes may combine them in a pot of mixed greens. Their different textures and flavors mean that substituting one for another changes the dish.

Mustard greens are also eaten raw in salads and used in cuisines beyond the American South. Their sharp taste is one reason cooks often balance them with fat, salt, acidity, or a little sweetness.