Cut horse mackerel fillets into bite-sized pieces and coat with potato starch, shaking off excess powder.

Thinly slice the onion and julienne the carrot.

Mix sugar, ponzu sauce, water, and sliced chili pepper in a large bowl to make Nanban sauce.

Heat salad oil in a frying pan until it covers the bottom. Fry the horse mackerel pieces over medium-low heat until golden brown on both sides. Remove and immediately immerse in Nanban sauce.