I got it from CantoDict originally. Then the other week, I came across this blog post which includes a link to a document claimed to be the official menu translations made by the Beijing Tourism Bureau in preparation for the 2008 Olympics. I grepped through the document and it uses "leek" to translate 韭菜 — I realise though that what they're calling a leek may not necessarily be the same thing that I'd understand as a leek.

I looked through my collection of transcribed menus, and the only one that (a) uses 韭菜 and (b) has English translations on the original is this one from Baozi Inn, which confirms that the item intended in this instance is Chinese chives.

I've edited the post to mention that the spring onion/leek thing may be inaccurate, pointing to this comment thread.