Cooking the soup. Combine the kombu dashi, potato, and abura-age in a sauce pot and quickly bring it to a boil. As soon as it comes to a boil, reduce the heat to simmer. Continue to cook until the potatoes are just tender. Test this with a bamboo skewer. The skewer should easily penetrate to the center of the potato with little resistance. As soon as they are, add in the soy sauce and the sake if you are using it. Finally stir in the nori and cook for one minute.
MISO TYPES. There are generally three types of soy based miso: white (shiro), yellow (shinshu), and red (aka). There are also different textures that you will come across, but the general differences between the three color categories are the main thing to understand for now.