1 large onion, thinly sliced
500ml/18fl oz whole milk (enough to cover the potatoes for cooking)
800g/1lb 12oz waxy potatoes (such as Maris Peer), peeled and thinly sliced
salt and black pepper
75g/2陆oz unsalted butter, plus extra for greasing
100ml/3陆fl oz double cream
4 large sheets filo pastry (about 25x40cm/10x16in each)
400g/14oz flaked, deboned hot-smoked salmon