Peel and slice the onion finely, and arrange half in the dish. Add a squeeze of lemon, salt and pepper. Put the fish on top, cover with the rest of the onion and another squeeze of lemon. Carefully pour in the cider.
Crumble the bread into crumbs and sprinkle on top of the fish. Dot with the butter.
Bake in a moderate oven (375F/190C/Gas Mark 5) until golden brown.