Peel potatoes and cut into very thin chips. A mandolin makes this easy; otherwise thinly slice, stack them and cut through the stack into sticks. Put the chips in a bowl and cover with cold salted water.
Cut the gravadlax into wide strips. Finely chop the onions. Melt a large knob of butter in a small pan, add the onion and fry for 5 mins. Butter the inside of a shallow ovenproof dish, about 1.2 litre capacity. Heat oven to 190C/fan 170C/gas 5.
Drain potatoes and spread one third over the dish. Season well. Scatter half the gravadlax and onion on top, then cover with another third of the potatoes. Scatter over remaining gravadlax and onion. Cover with remaining potatoes and season well. Warm cream and milk in a small pan until it simmers, then pour over the potatoes. Dot the top with butter and bake for 1 hr until potatoes are tender and the top golden. Cool for 5 mins before serving with a green vegetable such as broccoli or snap peas.