Cut the green onion into 6 equal pieces.

Pat the Spanish mackerel fillets dry with a paper towel.

Heat a frying pan over medium heat, add the green onion, and cook until browned. Flip, cover with a lid, and steam on low heat for 2 minutes. Remove from the pan.

In the same pan, add salad oil and heat. Place the mackerel fillets skin-side down and cook over medium heat for 2–3 minutes until browned. Flip, cover, and cook for another 2–3 minutes until cooked through. Add the mirin and ponzu sauce, spoon the sauce over the fish, and simmer until the sauce thickens.