Or, The Other Alliums

Shallots look like oval-shape, copper onions; the flesh is white and purple, but with a milder and sweeter flavor with a hint of garlic. There are usually 2 ‘cloves’ to a bulb. A staple of French cooking. A substitute for a shallot might be a combination of sweetish onion and garlic, approximately 2/3 onion and 1/3 garlic. A scallion is not acceptable when the recipe says shallot.

Scallions and green onions generally refer to exactly the same vegetable. They’re also sometimes called spring onions (Technically, there is a difference between the three, but this relates merely the size of the bulb). These are onions that have small bulbs and long green stalks; just a immature yellow onions actually. They’re much milder in flavor than ordinary onions, but a bit stronger than shallots. The greens are often used to add a sharp-sweet flavor and color to a dish. Popular in Asian cooking. If you need to substitute for the white bulb part, use a regular onion. For the greens, use chives.

Futhermore:

Chives are the smaller, more grass-like versions which are snipped finely and have a sharper taste than green onions. Don’t cook chives.

Leeks look like large green onions, and they have a more complex onion flavor. Much, much milder though. Must be washed thoroughly, as there is almost always dirt and sand between the layers.