Looking like an extra large scallion, the leek is related to the garlic and the onion even though its flavor and fragrance are milder and more subtle. Leeks are frequently cooked and served as a side vegetable or incorporated into soups and other cooked dishes. Leeks can be eaten raw as well but make sure just to use the whitest portion of the bulb. The wild version of the leek is called is called a ramp.
If you don't have leeks or just wish to find a substitute there are several good alternatives. For 1 cup of leeks substitute:
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