Horseradish Greens are the aboveground tops of Horseradish Plants. They have a bit of a bite to them, but nothing close to the bite that the root does. The young leaves are the best to use, when they are 3 inches long or less (7.5 cm.) Their flavour is quite mild. Larger than that, they get fairly coarse and develop a hotter flavour.
Horseradish Greens aren't sold. Only home gardeners have access to them.
You can sauté young Horseradish Greens, steam them, boil them or chop finely to add into salad. Some people use the older leaves as well, chopped up and cooked in with other greens.
In the Middle Ages, people commonly ate the leaves of Horseradish plants, in particular the Germans and the Dutch.