Cut the stems off the shiso leaves. Slice 5 leaves into thin strips and finely chop the other 5. Remove the pits from the plums and mash them finely.

Cut the mackerel fillets in half and coat them evenly with flour.

Heat salad oil in a pan over medium heat. Place the mackerel skin-side down and cook for 2 minutes. Flip and cook on low heat for 2–3 minutes until fully cooked. Remove from the pan.

Wipe the pan clean with a paper towel. Add water, sake, mirin, soy sauce, and mashed plums to the pan. Heat over medium heat until bubbling. Stir in the finely chopped shiso leaves and mix briefly. Add the cornstarch slurry and stir until the sauce thickens.