Chinese Mustard
Brassica juncea
Also known as Gai choy or heading mustard, this green is somewhere between Bekana mustard, bok choy, and Napa Cabbage. It comes with a sweet juicy stem and mildly pepper greens, great lightly steamed or stir-fried or tossed into some hot broth. Read more here, A Visual Guide to 10 Varieties of Asian Greens - kitchn, Beyond Bok Choy: Other Shades of Green - Red Kitchen
Photo - @anniesgarden
Chopping your veggies long and thin in a stir fry actually makes a huge difference! The increased surface area makes every bite pack more flavor.