What is 

Peppergrass, or Poor Man’s Pepper, is an annual plant that grows from a single taproot. It is in the Brassicaceae (mustard) family and it earned its name for the pepper-like flavour.
The entire plant is edible. Young leaves can be used as a potherb, sautéed or used fresh in salads. The young seedpods can be used as a substitute black pepper. The leaves contain protein, iron, vitamin A and vitamin C. The flowers can be tossed into a salad and the roots. This entire plant can be put into a food processor along with turmeric, vinegar, miso, garlic and salt to make wild mustard. Collect roots, wash them, crush them and add vinegar and salt you have a horseradish substitute.