Preheat the oven to 375 degrees F.
Slice the potatoes 1/8 inch thick, using a mandoline if you have one, or the slicer attachment of a food processor. In a bowl, toss the slices with the salt, then place them in a colander set over a bowl or the sink to drain for about 30 minutes. (It is important to rid the potatoes of any excess moisture that will make the pastry soggy during baking.) In a bowl, mix the potato slices with the onion, thyme, and butter, and set aside.
Line a baking pan with parchment paper. Lay the 10 by 8-inch sheet of pastry on a work surface. Spread the potato mixture evenly over the pastry, leaving a 1-inch border all around. (You might think that you have too much potato, but pile it on, as the mixture loses volume during cooking.) Brush the border with egg wash and top with the 11 by 9-inch sheet of pastry, stretching it over the potatoes.
Crimp the edges with a fork to prevent the pastry sheets from separating. Trim off any excess pastry and generously brush the top with egg wash. Poke a small vent hole in the center of the pastry to ensure an even rise. Crisscross the top with the back of the knife for decoration.
Bake until golden brown, about 60 minutes. Test the potatoes for doneness by inserting a small paring knife into the galette. If they are done, so is the pastry. If there is any sign of resistance, decrease the oven temperature to 300 degrees F and bake, testing frequently, another 15 minutes.
To serve, carefully transfer the galette to a large serving platter. Slice horizontally all around to separate the top and bottom crusts. Lift the top crust to expose the potatoes. Spoon creme fraeche evenly over them, reposition the top crust, and serve immediately. Slice the galette at the table.
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