Prepare the horse mackerel by removing the guts and spiny scute from the tail end.

Sprinkle salt evenly over both sides of the fish.

Grill the fish on both sides until cooked through.

Thinly slice the onion and julienne the carrot.

In a large frying pan, combine soy sauce, vinegar, sugar, and shiro-dashi to create the Nanban sauce.

Add the sliced onion and julienned carrot to the sauce.

Bring the sauce to a boil, then turn off the heat.

Once the fish is grilled, add it to the frying pan with the sauce.

Place the onion and carrots on top of the fish to infuse the flavors.

Flip the fish over in the sauce after 5 minutes to ensure even flavor absorption.

Allow the fish to simmer in the sauce for about 10 minutes to allow the flavors to meld.

Serve the fish with the Nanban sauce and vegetables once it has simmered to your liking.