Using a mandoline or sharp knife, slice potatoes about 1/8" thick.
Place in a very large bowl and add butter, thyme, salt and pepper, to taste and toss well, until all the slices are well coated with butter and seasoning.
Reserve about a dozen of the best slices for the top layer.
Arrange a layer of slightly overlapping slices in a 10 inch round gratin dish, skillet or other shallow oven-proof pan.
Layer potatoes until all are used and top with the reserved slices.
Place a sheet of heavy-duty foil directly on top of potatoes. Now take another heavy oven-proof pan, slightly smaller than the pan with the potatoes in it, and put it on top of the foil (this will act as a press).
Place in the oven and bake for 30 minutes.
Remove top pan, and carefully peel away foil. You may have to use a spatula to help separate the foil from the potatoes.
Continue baking until potatoes are soft and top is almost caramelized, 20-30 minutes longer.