There are many ways to make homemade soup seem thicker. Adding potatoes, beans or grains can balance out a thin broth but does not make a soup thicker. The best way to truly thicken the liquid component of the soup is to add a roux, which is a soup foundation made of equal parts flour and a fatty liquid like oil or butter. While it is best known as the initial ingredient for Cajun gumbo, you can add roux to any soup recipe to get a thicker consistency.
Things You'll Need
Fork or whisk
Oil or butter
Milk, if desired
Put a small saucepan on a burner at medium-low heat.
Add all-purpose flour and your preferred fatty liquid in equal amounts, increasing the volume as needed. Butter is ideal but you may also use cooking oil. If you are determined to avoid all the fat of butter or oil, use milk instead -- although the finished product will not be as flavorful.
Stir the mixture with a whisk or fork until all the flour dissolves and the consistency is smooth. Set the heat to low so the roux does not burn. Stir occasionally to keep lumps from forming.
Cook the roux until it reaches the consistency you want the soup to have. This will only take a few minutes. The roux will darken in color the longer you cook it. When using roux to make gumbo, cooks simmer it until it's the color of chocolate. The longer it cooks the more flavor it develops.
Add the finished roux to the pot of soup. Simmer the soup for several minutes until the roux thickens the broth.