In a saucepan, add the milk, creme fraiche, salt and potatoes (in very thin slices), and boil for 5 min.
Meanwhile, in a baking pan with removable bottom, lay the pastry sheets, oiling each one.
Keep 1 handful of the cheese mix and add the rest to the saucepan, together with the thyme and pepper, and stir.
Empty the mixture into the baking pan, cover evenly with the pastry sheets, and press lightly to remove the air. Carve the pie surface and bake in a preheated oven, at the bottom rack, at 170 oC for approx. 1 hr.
When the pie is half baked, sprinkle on top the cheese that you have kept aside and continue baking for another 30 min.
Take the pie out of the over and wait for 20 min to thicken. Finally, cut in pieces.