Drain and dry the potatoes and make as many layers as you can of potato and onion interspersed with the anchovy, sprinkling each layer with a little white pepper and ending with a layer of potatoes.
Sprinkle breadcrumbs over casserole and dot with the little cubes of butter.
Heat milk with the cream to a gentle simmer and pour over potatoes.
Bake in the centre of the oven for 45 minutes until tender when pierced with a sharp knife and the liquid is largely absorbed.