Brassica rapa var. parachinensis or Brassica chinensis var. parachinensis
Sometimes referred to as "Chinese flowering cabbage" (although all cabbages flower if left to their own devices) or Choy sum.
In Chinese it is, 菜心 (Mand: cài xīn; Cant: coi3 sam1) which means 'vegetable heart'.
Usually simply stir fried with garlic. Sometimes then finished with oyster sauce. It is also used in hot pots.
There is another version with slightly larger leaves and few flowers. This is known as 油菜心 (Mand: yóu cài xīn; Cant: jau4 coi3 sam1). To my palate, it tastes exactly the same as the first.
Finally, there is also a purple stemmed variety.
