"Mustard greens" refers to the edible leaves of the mustard plant. They are a type of vegetable that can be used in cooking and are often consumed for their nutritional value. Full definition
I was unsure what to do with a bunch of mustard greens I got from my CSA box, and this is a delicious solution. (epicurious.com)
This recipe would also be terrific with mustard greens in place of the spinach, although you'd have to cook the greens for a minute or two longer. (whatwouldcathyeat.com)
While bread is baking, toss mustard greens in a medium bowl with lemon juice and a generous pinch of salt. (bonappetit.com)