Mustard relative is a term used to describe three edible plants that are related to the mustard plant. The first is Cress, a leafy green vegetable with a peppery flavor. The second is Wasabi, a Japanese condiment made from the root of the Wasabia japonica plant. The third is Radish, a root vegetable with a spicy flavor. All three of these plants are related to the mustard plant and share similar flavors.