Chop the onion into chunks and stir-fry with some oil until translucent.
For the sweet potatoes, peel them and chop into cubes. Pour some hot water into the pan from Step 1 and add the potatoes. Boil until soft.
Once the potatoes have boiled, it's ideal if the water has boiled away too. Leave the potatoes to cool and add the soy milk (or milk) and white miso. Mix with a hand mixer or food processor until smooth.
Adjust the flavour of the soup with salt and pepper. Adjust the thickness with water or soy milk. When you want to enjoy it cold, add the salt and pepper before cooling. Top with a sprinkling of spring onion.
Story Behind this Recipe
I ate a sweet potato soup for lunch at a cafe and thought it was so good that I wanted to try and replicate it at home. The soup I ate tasted of seasoning and dairy products but I wanted to try making it "Ruruko"-style and made it non-dairy.
I used potatoes that were a bit old so they didn't have a very high water content. If you use newer potatoes you may end up with runnier soup. The taste was quite rich.