Make the crust:

• Make the crust. In a food processor, blend the flour, salt, and sugar together by pulsing several times. Alternatively, you can combine all these ingredients in a medium bowl with a whisk.
• Add the cold butter and zest and pulse to combine, or cut in by hand with a pastry cutter so that the mixture is crumbly with smaller than pea-size pieces of butter.
• Add 4 tablespoons of the cold water and pulse to combine, or stir with a wooden spoon to incorporate all ingredients. Continue to add cold water, one tablespoon at a time until dough begins to come together into a ball.
• Remove the dough from the processor (or bowl) and form into a ball, then press into a disc about 8” in diameter.
• Wrap the dough in plastic wrap and refrigerate for 2 hours or up to 2 days (or freeze up to a month).
• Preheat the oven to 375 degrees F.
• Once the dough is chilled, remove the disc from the refrigerator and place on a lightly floured work surface. Using a rolling pin, roll the dough to about 12” in diameter.
• Transfer the dough onto a piece of parchment paper big enough for the dough and baking sheet, approximately 12”x 15”. Set aside.
• Make the filling. Place the sliced sweet potatoes in a medium bowl and toss with the butter, garlic, thyme, salt, and pepper. I used disposable gloves to make it a little easier to get in between all those slices.
• Layer the potatoes in the center of the galette dough, leaving a 2 ½-inch border.
• Begin anywhere around the edge of the dough and fold the dough over the filling, so that the filling is encased by the dough. See main photo above.
• Make the egg wash. Blend the egg and water in a small bowl with a fork until the mixture is completely combined. Using a pastry brush, brush your egg wash on the edges of the dough that you just folded.
• Transfer the galette and parchment to the baking sheet. Place in the preheated oven and bake for 45 to 55 minutes.
• Begin checking for doneness of the potatoes and nice browning of the crust at 45 minutes.
• Depending on your oven, it may take more or less time. If your oven runs on the hotter side, you may want to begin checking at 40 minutes.
• Remove the galette from the oven and place on a cutting board or other flat serving surface. Allow it to cool for 10 minutes or longer if it is to be served at room temperature.
• Cut into wedges and serve.