Preheat oven to 400°F. Generously butter 13x9x2-inch glass dish. Arrange half of potato slices in bottom of prepared dish, overlapping slightly. Sprinkle generously with fleur de sel and freshly group pepper. Spread half of crème fraîche over potatoes and sprinkle with half of cheese mixture. Repeat with the remaingin potatoes, salt and pepper, crème fraîche, and cheese mixutre.
Bake gratin uncovered 30 minutes. Reduce oven temperature to 350°F. Continue to bake gratin until potatoes are tender and top is golden brown, about 45 minutes. If it is browning too quickly, cover with foil for the remaining baking time. Remove from oven; let stand 10 minutes before serving.