Gather your dry ingredients into a large bowl or the bowl of your food processor.
Use a pastry cutter or pulse the food processor about 5 - 6 times to cut the cold butter into your flour mixture until everything comes together into coarse crumbs the size of peas.
Add in the water one tablespoon at a time and mix. Add only just enough water that everything comes together when you press it with your hands.
Transfer everything to a sheet of plastic wrap and form into a disc. Wrap tightly and chill in the refrigerator for at least 30 minutes.
Place the chilled dough onto a lightly floured surface and roll out with a rolling pin until it is about the size of a 12-inch circle. It does not need to be a perfect circle.
Add the filling and leave a 1 to 2-inch border so you can fold the border over the filling.
Brush the folded over dough with an egg wash and bake until golden brown
Notes
You might not need all of the water, depending on the humidity of your kitchen.
Dough can be made ahead of time and left in the refrigerator for up to 3 days or in the freezer for up to a month. Thaw in the refrigerator.
Allow the dough to rest for a few minutes at room temperature before rolling out to minimize shrinkage or cracks in the dough. Use your fingers to press together any cracks.
Leave the galette with the fillings in the freezer while the oven preheats. Only brush with the egg wash and top with your sugar or anything else you may use just prior to placing in the oven.