This is a light but filling soup that's easy to make and tasty. Note: This is a salty soup that can be made with white miso, but if you mix red and white, the flavor is superb. Some cooks drop an egg into the soup as it cooks, and/or add mushrooms or slices of white radish or potato. It can be served by itself or poured over a bowl of rice to make a gruel, accompanying a dish of fish and yellow Japanese pickles called takuan.