Add water a little at a time. This can be quite tricky as the water instantly changes the runny peanut butter properties. I would say it is a similar experience to white sauce, when the milk is added to the flour and butter mixture. Once a smooth sauce is achieved, allow to simmer. You will find the peanut oil begins to separate from the peanut butter. When this happens, you can add the peanut butter sauce to the rest of the soup and simmer for 20 minutes till oil rises to the surface of the soup.