flour to use when thickening a soup
6. Make a corn, rice, or potato paste. Mix cornflour, rice flour, or potato flour with water until you have formed a thick paste. Whisk this into the soup until the soup thickens. You can also use wine, stock, or other liquid in place of the water for making the paste.. Use cornstarch. This method will thicken the soup without as much additional cook-time as the flour method. Put about a tablespoon of starch in a container and add just enough cold water to make a thick slurry or paste.