Zosui is a mild and thin Japanese rice soup. The rice are cooked before putting into a soup which uses fish broth and soy sauce or miso paste. Also vegetables, mushrooms, meat or seafood are added. These ingredients are chopped much smaller sizes than miso soup and zosui usually include egg.
Zosui is most likely eaten after finish eating nabe hot pot at home or restaurant. In this case, there is no extra fee for zosui usually. Then, it can be found as a single meal at restaurant or tavern during winter.
The food is sometimes called ojiya too. However, many people recognize zosui and ojiya are similar but different food. Anyway, restaurants serve zosui but hardly to find ojiya on the menu. Probably zosui is a proper meal which can be provided by charging to customers. On the other hand, ojiya is a dish which is too simple and anyone can cook it at home.