Put the potatoes in a microwaveable bowl with a splash of water, cover, then cook on high for 5 mins until tender (or steam or simmer them).
Meanwhile, heat the oil in a frying pan over a medium heat, add the leeks with a pinch of salt and cook for 10 mins, stirring so they don’t stick, until softened. Tip in the potatoes, turn up the heat and fry for a couple of mins to crisp them up a bit. Flake through the mackerel.
Make four indents in the leek mixture in the pan, crack an egg into each, season, then cover the pan and cook for 6-8 mins until the whites have set and the yolks are runny. Serve the horseradish on the side, with the pan in the middle of the table.