Zosui

Zōsui , or ojiya (おじや), is a mild and thin Japanese rice soup akin to a rice-based vegetable soup. It is made from pre-cooked rice and dashi or water seasoned with either soy sauce or miso and cooked with other ingredients such as meat, seafood, mushrooms, and vegetables. It is generally served to those who are sick or otherwise feeling unwell, and is usually only served in the winter. Leftover soup from nabe is often re-used for zosui. Instead of rice, udon and ramen noodles are recent alternatives. In the days when it was difficult to keep cooked rice warm, the only way to reuse cold rice was to combine it with miso soup, so this was widely done in households across Japan. Nowadays, it is more often used to make meals for the sick or those feeling unwell rather than in everyday meals.

Source: Wikipedia