Preheat the oven to 450 degrees F/230 degrees C.
Spread the creme fraiche over the pastry, going all the way out to the edges. Scatter the onion slices and bacon over the creme fraiche. Grind some pepper on top and sprinkle with fleur de sel. Bake until the creme fraiche is bubbling, the bacon is cooked and the onions are curled and starting to brown, about 20 minutes.