Spring onion soup is a dish that either highlights the flavor of spring onions or uses them as one of its main ingredients. The soup can be a very simple and thin broth made from stock and spring onions, or it can be much heavier and include cream, potatoes or yogurt. The flavor of spring onions often is paired with similar vegetables, such as yellow onions or leeks, to provide more depth. In some instances, spring onions are used in place of pure onions to help tone down the taste of a dish, making it possible to create milder-tasting versions of meals such as French onion soup. Depending on the ingredients, a finished spring onion soup can be served with croutons, yogurt, sour cream or other garnishes.
In the United States, a spring onion is usually considered a variety of allium separate from a green onion or scallion. The bulb on the end of a spring onion is generally larger than a scallion, and the taste is sharper, being closer to a yellow onion. A spring onion has two portions that can be used in cooking, the crisp white bulb and lower pale end of the leaves, and the more fibrous tops of the green leaves. The white portion is almost always used for spring onion soup, while the green portion is sometimes added as a garnish at the end of the recipe. Both portions are edible, with the green leaves having a much more subdued flavor than the white area.
One of the simplest recipes for spring onion soup that accents the flavor of spring onions is made by frying chopped, cleaned spring onions in olive oil. Once they have become translucent, the onions are boiled for a time in vegetable or chicken stock and some water. After the flavors have merged, the stock is placed in a food processor or blender and the entire soup is pureed into a thick liquid that can be served in a bowl and drizzled with olive oil.
The sharp taste of spring onions has led to some recipes that create a cream-based spring onion soup to help mellow the flavor. Spring onions are usually fried until soft and then added to stock along with heavy cream, sour cream or buttermilk before being reduced to make a hearty soup. Potatoes can be added for thickness, and it is not uncommon to find leeks, garlic or yellow onions used in large amounts.