The leaves of the mustard plant (Brassica juncea) are hot and peppery tasting, and can be yellowish-green or reddish-purple in color. As with other hardy greens like collards or turnip greens, they require longer cooking time to make their fibrous texture tender.
Their pungent flavor is flattered with garlic, hot pepper flakes, and vinegar, or long cooking with a ham hock.
Don't have it?
Kale and turnip greens are also hardy, leafy vegetables with a similar flavor profile (minus the horseradish-y bite), and they would work in most recipes that call for mustard greens.
How to choose:
Choose leaves that look fresh rather than wilted or drooping, without any yellow spots or bruising.