Trim the ends and remove the calyx of the okra. Sprinkle with 1/3 tsp salt and rub on a cutting board. Boil water in a pot, add the salted okra, and cook for 30 seconds. Rinse under cold water, drain, and slice into 7mm pieces. Cover the nagaimo with plastic wrap and crush it finely using a rolling pin or similar tool.

Mix the natto with its included sauce.

Pat the tuna dry with kitchen paper and cut into 1.5cm cubes.