Preheat the oven to 190C/gas 5/fan 170C. Boil the potatoes in their skins for 12-15 minutes until nearly cooked through. Drain and slice into thin discs. Heat the oil in a large frying pan and cook the onion until soft and golden. Gently toss the potatoes and onion together and season with salt to taste.
Put half the mixture in a shallow 1.4 litre ovenproof dish. Remove the skin and any bones from the mackerel, flake the fish and spread it out over the potatoes. Top with the remaining potatoes and pour over the cream.
Cover with foil and bake for 30 minutes, then uncover and bake for a further 20-25 minutes, until the topping is a lovely golden brown.