Thaw the frozen Spanish mackerel, rinse lightly under running water, and pat dry with a kitchen towel.

Marinate the mackerel with 2 tbsp mirin, a pinch of black pepper, ginger powder, and salt for 5-10 minutes. Pat dry again with a kitchen towel after marination.

Cut garlic cloves in half and set aside.

Coat the mackerel evenly with cornstarch.

Heat a pan with generous cooking oil and fry the mackerel until golden brown on both sides. Add garlic halfway through and fry until golden.

In the same pan, combine 3 tbsp soy sauce, 1 tbsp sugar, a pinch of ginger powder, black pepper, 3 tbsp mirin, and 2 tbsp water. Simmer over low heat.

Add the fried mackerel and garlic to the sauce, simmering until the sauce is absorbed and coats the fish evenly.