Heat the ove to Gas Mark 4.

Lightly grease an ovenproof dish and place a layer of sliced pototes on the bottom, then a layer of onions and repeat until finished with a layer of potatoes.

Pour over the stocka nd milk mixture, sprinkle with thyme and dot with small pieces of butter and bake fir 30 minutes.

Cover with foil and bake for a further 40 minutes.

Remove foil and cook for the final 10 minutes.