Thinly slice the onion and julienne the carrot. Mix the sauce ingredients (vinegar, sugar, salt, soy sauce, dashi powder) in a container, then add the vegetables and let them marinate.

Remove small bones from the horse mackerel fillets and cut them into bite-sized pieces. Coat the pieces with potato starch.

Heat salad oil in a frying pan over medium heat. Pan-fry the fish pieces, flipping them occasionally, until golden brown.