Tatsoi Brassica rapa var. rosularis is a Chinese green (a type of bok choy) whose leaves can be eaten raw or cooked. The leaves are tougher and stronger flavored than the standard pak choy. The green is typically stir-fried, steamed or used in soups. Only the young leaves are eat raw in salads.
The plant grows horizontally rather than upright like the more familiar forms of bok choy. The leaves look more like spinach than boy choy. If you grown your own you can harvest individual leaves at a time rather than pulling up the whole plant. You can also use the seed to grown your own microgreens. A similar, related green is Joy Choi which is easy to grow too. You can purchase the seed for either of these plants at Kitazawaseed.com.
If you don't have tatsoi then there are several good alternatives:
Look for this green in Asian markets. Both Japanese and Chinese have a version of this green. It can be grown in winter or summer (in some climate zones) so growing your own might be an option.