If in doubt, always call your vet or the Animal Poison Line immediately.
Leeks are in the same plant family as onions and garlic and contain the same damaging thiosulphate compounds. All parts of the plant are toxic : the bulb, stalk, and leaves. Cooked leeks in soups and casseroles are just as harmful.
Contact your vet, especially if your dog has eaten leek-containing food such as soups or stews.