1. Scrub the potatoes, drop them into boiling salted water and cook for 30 minutes. Soak the herrings in water for 30 minutes, changing the water several times. Then skin, bone and dice the herrings. Peel the potatoes and cut them into fairly thick slices. Chop the onion.
2. Melt 50 g (2 oz) of the butter in a saucepan, add and fry the onion until transparent, then sprinkle on the flour and cook until golden, stirring continuously.
3. Gradually stir in the water and simmer gently for 5 minutes. Remove the sauce from the heat and stir in the cream. Dice the ham and stir it into the sauce.
4. Arrange the potato and herring in layers in a buttered ovenproof dish, finishing with a layer of potato. Pour on the sauce, sprinkle with the cheese and dot with the remaining butter.
5. Bake in a moderately hot oven (200c, 400f, gas 6) for 30 minutes.
Serve with: beetroot salad.