Tatsoi (Brassica narinosa), also tat soi, tatsoy, tat soy, is a variation of field mustard originally cultivated in Asia but now increasingly popular as a salad green or cooking ingredient throughout the world.
Known as ta1 cai4 (塌菜) in Chinese and taasai in Japanese, the plant is officially known in English as "broadbeaked mustard" according to the USDA, but that term yields only 469 hits on Google. More common English renderings include spinach mustard, spoon mustard, or rosette bok choy, although by far the most common term is some variation of "tatsoi."
The various English names all speak to different aspects of the plant, with its Asian origins, it's resemblance to a large bundle of bok choy, its spoon-shaped leaves, and its taste, which is often described as a fusion of spinach and mustard greens.
Tatsoi is an outstanding choice for salads, and also makes for excellent stir fry. Not to mention that as a form of mustard it is chock full of vitamins including Vitamin A, Vitamin C, Thiamin, Riboflavin, Vitamin B6, Folate, Calcium, Iron, Potassium, and Manganese.
Altogether a very tasty and still often overlooked vegetable.