Add the creme fraiche, chicken stock paste, reserved potato water and the cheese. Mix together, bring to the boil, then remove from the heat. Season to taste.

Lay the cooked potato slices in layers in an appropriately sized ovenproof dish, then pour over the creamy sauce. Bake on the top shelf of your oven until golden brown and bubbly, 15-20 mins. TIP: Put the dish onto a baking tray to catch any drips.