Cut the potatoes in half, then into quarters. Soak in water and drain. Place in a microwave-safe container, add 1 tbsp water, cover loosely with plastic wrap, and microwave at 600W for 5 minutes. Drain and mash while hot.

Thinly slice the onion, sprinkle with a pinch of salt, and let sit for 5 minutes. Rinse with water and pat dry with a paper towel.

Drain the canned mackerel and lightly flake it, leaving some chunks intact.