Sprinkle salt on both sides of the mackerel and let it sit for 5 minutes. Pat dry with paper towels. Cut into bite-sized pieces and coat both sides with pepper and flour.

Cut the potato into bite-sized pieces and soak in water to remove excess starch. Crush the garlic clove with the flat side of a knife.

Place the potato in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 2 minutes. Drain any excess water.

Heat salad oil in a frying pan over medium heat. Add garlic and cook until fragrant. Add the mackerel, skin-side down, and cook until golden brown. Flip and cook through, then remove from the pan. Add the potato to the same pan and cook until both sides are golden and tender.