It belongs to the same Brassica Rapa family as pak choy, tatsoi, napa cabbage etc. Its subspecies is Parachinensis. It is called ‘sawi caisim’ among the locals which is a mix of Malay and Mandarin Language. 菜心 means heart vegetable.
Some places call it ’油菜’ (yóucài in Mandarin & ‘yu choy’ in Cantonese) which means oil vegetable. The oil refers to oil seed to produce cooking oil e.g. rapeseed oil or canola. However, some areas also refer to pak choy as yóucài. The confusing part is 油菜花 (oil vegetable flower) is actually another plant that belongs to the Brassica Napus family1.
There are many types of choy sum e.g. Shinta, Kumala, Saina etc. but I would just divide it into 2 main ones: Thick stem (HK) and thin stem (regular) ones.
Hong Kong Choy Sum/ 香港菜心/ Hong Kong Sawi
It has a thicker stem and looks a bit like Kailan but has yellow flower and a different texture. It is served whole in Chinese restaurant drenched in sauce. It is sweeter, less bitter and crunchier than the regular choy sum. And it is double the price compared to regular choy sum.
Choy Sum/菜心(càixīn)/ Sawi Hijau & Sawi Bunga
The (regular) choy sum has thin stem and both of them form my local supermarket taste quite similar.
My supermarket refers to the bigger one as ‘sawi hijau besar’ which means big green mustard leaf.
Cook Choy Sum
It is a very versatile vegetable to cook Asian dishes. I boil it and put it in my noodle soups or use it to fry rice or noodles with it. You can stir fry and boil it.
Hong Kong Choy Sum with Oyster Garlic Sauce
Choi Sum Namul
Stir Fried Choy Sum in lei cha
Choy Sum in Fried Rice
Choy Sam in Fried Beehoon
Boiled Choy Sum in Beef Noodle Soup
Choi Sum in Meatball and Fishball Soup
Choy Sum in Pasta Anchovy Soup
I used to eat this pasta anchovy soup for my lunch as a child. I used to like pasta and anchovies which explains why this combination was formed.