The whole prefecture

Natto, tofu, fried tofu, edible wild plants, Imogara(made from dried stems of Karatori potatoes), konjac, mushrooms, onions, Japanese parsley, miso

In Yamagata Prefecture, it was common for people to make natto (fermented soybeans) at home as a way of preserving food to get through the winter when foodstuffs are in short supply. Natto was made by stuffing boiled soybeans into a tube made of straw and placing it in a warm place such as beside the kotatsu (heating table covered with a blanket) for one or two nights. In the Murayama area of Oe Town, fermented soybeans without strings are made into "Natto Jiru” (natto soup). “Natto Jiru” is a winter home-style dish that has been popular in Yamagata Prefecture for a long time. Natto is milled in a mortar until it becomes completely creamy, then thickened and blended into the soup. “Imogara”, made from dried stems of Karatori potatoes, is an indispensable ingredient in “Natto Jiru”. It was a valuable source of protein with plenty of tofu and fried tofu, along with dried and salted products and other preserved foods. It is eaten on the day of Nanakusa (January 7) in the Murayama area, on New Year's Day in the Mogami area, and in the Shonai area, it is sometimes eaten on "Daikoku-sama no Otoshiya (New Year's Eve of God of Wealth)" (December 9).

Natto soup is made from natto (fermented soybeans), which thickens the soup and keeps it from cooling down, and has been eaten as a warming soup in cold winters.
In regions with deep snow, it is not possible to prepare the seven herbs in early spring, so it is customary to eat natto soup made from dried and preserved food during the seven herb season to pray for good health and good fortune throughout the year. In the Mogami area, it is eaten on New Year's Day and January 3. In the Shonai area, it is eaten on December 9, the night Daikoku-sama welcomes his wife, or on New Year's Eve. On "Daikoku-sama no Otsu-ya," fried beans, fried rice, and "makka daikon" (radish with two halves) are offered, and dishes made with beans and daikon are laid out to celebrate a bountiful harvest and prosperity of the offspring.

The coarseness of the mashed natto varies from household to household, but it is often mashed so carefully that there is no natto in it at first glance. By mashing, the natto is easily absorbed into the soup and becomes smooth to the palate. The key to cooking natto is to mash each bean individually, because if you stir it too much, it becomes sticky and slippery, making it difficult to mash.
The secret to cooking natto is to mash the beans one by one. When adding the mashed natto, combine it with miso in a mortar and pestle, and add it after spreading it in the soup to make it easier to mix. Once miso and natto are added, it is best not to overcook them.

Recently, it is less commonly made at home due to the time and effort required, but it is still served at inns, restaurants, and as a school lunch menu item. Natto soup is one of the most popular items on school lunch menus.
To make natto soup easy to prepare at home, it has been commercialized as "natto soup set (natto soup stock)" and is sold at supermarkets.