What's this green vegetable with no leaves used in Cantonese cuisine?

That appears to be gai lan, or a closely related Chinese brassica like yu choy, with the leaves removed. So just the stems.

I haven't seen (or found) a recipe for just the stems in broth, but it makes a certain amount of sense; the leaves cook much faster than the thick stems, so sometimes just the leaves are used for stir-fry. Then you can do this with the stems.

Probably leeks. Almost all Asian meals use them. Similar to onions or cabbage

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