Place rack in center of oven and preheat to 400 degrees. Generously butter 13x9" glass or ceraminc baking dish.
Whisk creme fraiche and whipping cream in medium bowl to blend.
Arrange 1/3 of potato slices in bottom of prepared baking dish, overlapping slightly. Sprinkle with 1/2 tsp salt, 1/4 tsp pepper and 1/2 tsp thyme. Drop 1/3 of cream fraiche mixture in dollops over potatoes, then spread evenly over (layer will be very thin). Repeat layering 2 more times.
Bake gratin 30 minutes. Reduce oven temperature to 350 degrees and continue to bake until potatoes are tender and top is golden brown, about 25 minutes more. Let gratin rest 20 minutes before serving.
Can be made up to 2 hours ahead. Cover loosely with foil. Let stand at room. Before serving, rewarm in 350 degree oven until heated through, about 15 minutes.