It is called Chanko-nabe which is cooked by sumo wrestlers as nabe-ryori. There are basically two kind of flavors; mizu-taki (chiri-nabe) and shio-taki (yose-nabe). Chanko-nabe has been eaten for almost 200 years at sumo gyms. It is cooked with water and put chicken, mushrooms, cabbages, green onion, and so on. You can add any seasonings such as soy sauce, sugar, or sake so that you can eat different Chanko flavor everyday like sumo wrestlers do.
It may give you various opportunities to learn Japanese cultures through making your own Chanko-nabe and learning about sumo.
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