Toriten is a local dish in Oita Prefecture and each household is likely to have a slightly different recipe but most of the recipes use egg in the batter.
If you deep-fry the dish with a small amount of oil, the oil temperature will easily change so you should adjust the heat frequently.
Kabosu, a type of sour citrus is also one of the Oita’s specialties. Ponzu sauce using the kabosu juice along with karashi also goes great with Toriten. If kabosu isn’t available, use your local sour citrus, for example lemon, yuzu or lime instead.