Mix mirin, pepper, ginger powder, and salt. Rub the mixture onto the mackerel and let it marinate.

Slice garlic into thick pieces and set aside.

Combine soy sauce, mirin, plum syrup, corn syrup, sugar, and water to make the sauce.

Place flour and cornstarch in a bag, add the marinated mackerel, seal the bag, and shake to coat the fish.

Heat oil in a pan and fry the mackerel until golden brown on both sides. Drain excess oil on paper towels.

Pour the sauce into a pan and bring it to a boil. Add garlic and cook until slightly softened.

Add the fried mackerel to the sauce and simmer until fully coated and cooked through.