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.