Japanese leek (naga-negi)
A long, thick Welsh onion with a tall white shaft, mildly pungent and sweet once cooked.
Naga-negi is sold in Asian grocers, and thick Welsh onions also turn up in Turkish or Middle Eastern shops. Ordinary supermarkets usually carry only the thinner spring onions.
Substitute:
For cooking, a young, slender leek works, though it is more fibrous and needs longer. Raw as a garnish, use the white and pale green parts of sturdy spring onions, which are a little sharper and less juicy.