Preheat oven to 170¬8C. Brush a 2.5L (10-cup) capacity ovenproof dish with melted butter to lightly grease. Combine the cream and milk in a saucepan over low heat and cook, stirring occasionally, for 2 minutes or until heated through.
Arrange half the potato slices over the base of the prepared dish. Sprinkle with half the onion. Season with salt and pepper. Pour over half the cream mixture. Continue layering with the remaining potato slices, onion, salt and pepper and cream mixture.
Cover with foil and place on a baking tray. Bake in oven for 1 hour or until potato is tender. Scatter prosciutto over the potato bake and sprinkle with Parmesan. Bake, uncovered, for a further 25-30 minutes or until golden brown and the potato is very tender.
Set aside for 15 minutes to cool. Serve.