Clean the leek and cut the white and light green into rings. Halve the potatoes diagonally or, depending on the size, quarter them, mix with the leek, season with salt and pepper. Put in an ovenproof dish. Salt and pepper 200 ml of cream. Add the dried thyme. Pour over the potatoes.
Bake in the preheated oven at 200 degrees on the 2nd rack from the bottom for 30 minutes. If you want, you can sprinkle some grated mountain cheese over it before gratinating. Tastes very tasty too.