Slice the onion and carrot thinly and set aside.

Season the horse mackerel fillets with salt and pepper, then coat them lightly with flour.

Heat olive oil in a pan and fry the fillets until golden brown. Remove and let cool.

Prepare the marinade by mixing water, sugar, salt, and vinegar in a bowl.

Layer the fried fillets, sliced onion, and carrot in a container, then pour the marinade over them.