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1
Preheat the oven to 375 degrees F.
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2
Slice the potatoes 1/8 inch thick, using a mandoline if you have one, or the slicer attachment of a food processor.
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3
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.
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4
(It is important to rid the potatoes of any excess moisture that will make the pastry soggy during baking.)
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5
In a bowl, mix the potato slices with the onion, thyme, and butter, and set aside.
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6
Line a baking pan with parchment paper.
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7
Lay the 10 by 8-inch sheet of pastry on a work surface.
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8
Spread the potato mixture evenly over the pastry, leaving a 1-inch border all around.
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9
(You might think that you have too much potato, but pile it on, as the mixture loses volume during cooking.)
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10
Brush the border with egg wash and top with the 11 by 9-inch sheet of pastry, stretching it over the potatoes.
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11
Crimp the edges with a fork to prevent the pastry sheets from separating.
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12
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.
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13
Crisscross the top with the back of the knife for decoration.
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14
Bake until golden brown, about 60 minutes.
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15
Test the potatoes for doneness by inserting a small paring knife into the galette.
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16
If they are done, so is the pastry.
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17
If there is any sign of resistance, decrease the oven temperature to 300 degrees F and bake, testing frequently, another 15 minutes.
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18
To serve, carefully transfer the galette to a large serving platter.
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19
Slice horizontally all around to separate the top and bottom crusts.
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20
Lift the top crust to expose the potatoes.
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21
Spoon creme fraeche evenly over them, reposition the top crust, and serve immediately.
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22
Slice the galette at the table.
