Whisk together all of the ingredients except the potato slices.
Make a layer of potato and sweet potato slices in a deep baking dish and pour over some of the cream mixture. Repeat until all of the ingredients are used up.
Cover with aluminium foil and bake at 180°C for 35-45 minutes.
Remove the foil and continue to bake until golden brown. Allow to rest for at least 10 minutes before serving.