Preheat the oven to 180 C gas Mark 4. Grease a 20 cm gratin dish with the butter. Skin the mackerel and flake the flesh into bite sized pieces, discarding any bones.
In a bowl gently mix together the fish, onion and potatoes then put in the gratin dish.
Mix together the milk, cream and mustard , season with salt and pepper then pour over the potatoes. Cover the dish with foil and bake for 45 minutes.
Remove the foil and continue to cook for 10-15 minutes until the top is golden brown, the cream is bubbling and the potatoes are tender. Serve immediately.