Trim the stems off the shiso leaves. Cut the cheese slices in half. Crack the egg into a bowl and mix to create egg wash.

Mix breadcrumbs and black sesame seeds in a tray to create sesame breadcrumbs.

Pat the horse mackerel fillets dry with paper towels, season with salt and pepper, and coat lightly with flour.

Place one shiso leaf, half a slice of cheese, and another shiso leaf on the skin side of a fillet. Sandwich with another fillet, skin side up. Repeat for all fillets.

Dip the prepared fish sandwiches into the egg wash, then coat them with sesame breadcrumbs.