Yet another addition to the bok choy confusion. This is a Sichuan cultivar known as 四川儿菜 (sì chuān ér cài), literally Sichuan young vegetable.
These are about 10 cm / 4 inches in height and taste mild but indentifiably cruciferous. Usually halved lengthwise and stir fried with garlic and soy.
It isn't a Sichuan treatment but I like this one with a dribble of oyster sauce.