Watercress is a peppery, aquatic green from the mustard family, with the same mustard-oil bite you'd find in arugula or radish but cleaner and more grassy. The leaves are small, round, and tender, perched on crisp, hollow stems that snap rather than bend. Flavor lands in two waves: an immediate fresh-green, almost cucumber-cool note, followed by a slow horseradish-like heat that lingers at the back of the palate.