A leek is a vegetable that looks like a giant green onion with a long white stalk and flat green leaves. The white and light green parts taste mild and slightly sweet when cooked, much gentler than the sharp bite of a regular onion.

Leeks grow partly underground, which means dirt gets trapped between their many layers. Cooks slice them lengthwise and rinse them carefully under running water before using them. They're delicious in soups (especially the classic potato-leek soup), stir-fries, and casseroles, adding a subtle onion flavor without overpowering other ingredients.

The leek has been cultivated for thousands of years and appears in cuisines around the world. It's the national symbol of Wales, where people traditionally wear leeks on St. David's Day to celebrate Welsh heritage.

When you see leeks at the grocery store, they're usually bundled together and look somewhat floppy, with roots at one end and long leaves at the other. Though they take up more space in your shopping cart than a regular onion, their mellow flavor makes them worth the extra room.