Cut the green onion into 3cm pieces, the red bell pepper into bite-sized pieces, and separate the maitake mushrooms into small clusters.

Pat the mackerel dry with a paper towel, season with salt and pepper, and coat with all-purpose flour.

Heat half of the sesame oil (1 tsp) in a frying pan over medium heat, add the green onion, red bell pepper, and maitake mushrooms, and cook until browned. Remove from the pan.

Add the remaining sesame oil (1 tsp) to the pan, heat over medium heat, and place the mackerel skin-side down. Cook for 1 minute, flip, and cook for another 2 minutes until browned. Remove excess oil with a paper towel.

Add the sauce ingredients (sake, sugar, soy sauce, oyster sauce, and water) to the pan and bring to a boil. Add the cooked vegetables, cover, and simmer on low heat for 4 minutes.