Butter a leg grating dish. Preheat the oven to 375°.

Wash and peel the potatoes and slice very thinly. Peel and quarter the apples and cut into thin wedges. Sprinkle immediately with lemon juice to prevent browning.

Arrange the potato and apple slices in the grating dish in alternate layers, sprinkling each layer with salt, pepper and a little thyme.

Finally pour the cream over and top with sliced Brie. Dot with flakes of butter and bake on the middle shelf of the oven for 45 minutes.