Make the dashi broth according to package instructions of your instant powder.
Peel the nagaimo and grate it using a ceramic or cheese grater. I recommend using a glove or quickly washing your hands after, as it irritates the skin.
Combine flour, salt, sugar, and baking powder in a large bowl.
Add in the grated nagaimo and dashi, mix thoroughly.
Store it in the fridge.
Wash the cabbage, then slice into pieces of 1-2cm width and length. To achieve this, remove the tough stem, cut into slices and then slice them again lengthwise.
In a large bowl, thoroughly mix the coarse salt into the cabbage.
Let the batter and cabbage rest for at least one hour.