Negi (ネギ, ねぎ), sometimes called Naga Negi (long onion 長ネギ), Shiro Negi (white onion 白ネギ), or Tokyo Negi (東京ネギ), is the Japanese name for Allium fistulosum (Welsh onion). It’s longer and thicker, and has a larger amount of white stem.
The white part has a strong onion taste, but when it’s cooked, it becomes sweeter. This part is used in many braised and simmered dishes as well as nabemono (hot pot) dishes. The green part is used just like how we use scallions/green onions.
Substitute: Leeks can be used for similar texture, but scallions/green onions should be added for the similar taste of negi.