"Mustard greens, where have you been all my life?! The earthy, mild onion flavor of leeks combined with the spicy kick of mustard greens braised in a buttered broth makes for a simple and tasty autumn side dish."
Trim the leeks so that only 1 1/2 inches of green remain, then slice each stalk lengthwise. Slightly pull the leaves apart and rinse well in cool water to remove all of dirt and sand from between them. Chop each halved stalk into 1 1/2 inch pieces.
Saute the leeks in the butter, stirring frequently, for about 5 minutes, or just until the leeks begin to soften. (Some of the leaves will separate.)
Add the chopped mustard greens, and just enough chicken broth so that it covers the bottom of the pan (goal is to to keep the leeks from browning). Saute the leeks and mustard greens until the mustard greens just begin to soften (2-3 minutes).
Remove from stove top, add salt and pepper to taste, and sprinkle with parmesan cheese.