Pat the mackerel fillets dry with a paper towel and score the skin with cross cuts.

Slice the green onion diagonally and cut the chives into 3cm pieces.

In a pot, combine sake, mirin, soy sauce, and water. Bring to a boil over medium heat, then add the mackerel skin-side up. Spoon the simmering sauce over the skin to cook evenly.

Add kimchi and green onion to the pot. Cover and simmer for about 10 minutes until the mackerel is cooked through.