Japanese leek
Japanese long onion (negi), tall and slender, somewhere between a leek and a spring onion, with a long white shaft and a mild flavour that turns sweet when cooked.
Asian grocers sell it fresh or frozen; ordinary supermarkets rarely carry it.
Substitute:
Thin young leeks or sturdy spring onions, mainly their white parts, come very close. Leeks are a little more fibrous, spring onions a little sharper.