All About Allium Vegetables. With the recent press on alliums such as onions and garlic protecting against both cardiovascular disease and cancer, we asked a friendly vegetable expert to tell us more about this family of vegetables.heir flavor, sometimes described as a blend of sweet onion and garlic, make them a favorite of gourmet chefs. Leeks (Allium ampeloprasum var. porrum) are the largest member of the allium family and look like gigantic scallions. They may grow up to two feet long and two inches thick, and they do not form a bulb.