Slice the garlic thinly and remove the core. Cut the potato in half, then slice into 5mm thick pieces. Soak in water and drain.

Cut the mackerel into 1.5cm wide pieces and sprinkle with sake. Let sit for 5 minutes, then pat dry with paper towels and coat with flour.

Heat olive oil and garlic in a frying pan over low heat until fragrant. Add the mackerel skin-side down and cook over medium heat for 2 minutes. Flip and move to the side of the pan.

Add the potato slices to the pan and sauté until translucent. Cover with a lid and steam over low heat for 2 minutes until cooked through. Add salt and black pepper, then lightly stir.