Make the crust:

• Make the crust: Ina food processor, blend the flour, salt, and sugar together by pulsing several times. Alternatively, you can combine all these ingredients in a medium bowl
• Add the cold butter and zest and pulse to combine, or cut in by hand or with a pastry cutter so that the butter is pea-size.
• 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 shaped 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.
• Once chilled, remove the dough from the refrigerator and place on a lightly floured work surface. Using a rolling pin, roll the dough to about 12” round.
• Transfer your rolled dough onto a piece of parchment 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, and thyme. I used disposable gloves to make it a little easier to get in between all those slices.
• Once all the ingredients for the filling are combined, layer the potatoes in the center of the galette dough, leaving a 2 ½-inch border to fold over.
• Begin anywhere around the edge of the dough and fold the dough over the edge of filling, folding all the way around 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 for until the mixture is completely combined. Using a pastry brush, brush your egg wash on the surface of the dough that you just folded over.
• Place in a 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 to cool for 10 minutes or longer if to be served at room temperature.
• Cut into wedges and serve.