Use a fork or pastry cutter to cut the butter into the flour, stopping when the mixture resembles course crumbs.
Mix in water, one tablespoon at a time, adding just enough for dough to clump together in a rough ball (typically 3 to 4 tablespoons total).
Shape dough into a flat circle about 1-inch thick. Wrap in plastic wrap, then refrigerate for at least 1 hour.
Prepare Peaches:
Combine peaches, brown sugar, cornstarch, cinnamon, lemon zest (save juice for another use), and salt in a large bowl. Mix well, then set aside.
Assemble Galette:
Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper, then set aside.
On a lightly floured surface, use a rolling pin to roll pastry into a 12-inch circle. Transfer to parchment-lined baking sheet.
Arrange peaches over pastry, leaving a 1 ½ to 2-inch wide border empty.
Fold pastry edge over filling, overlapping where needed.
Whisk together egg yolk (save white for another use) and 1 tablespoon water. Brush egg wash over pastry.
Sprinkle coarse sugar over egg-washed pastry.
Bake galette until dough is deep gold brown and crisp (about 40 to 45 minutes).
Let galette rest for 20 to 30 minutes, then cut into wedges. Serve warm (or at room temperature) topped with ice cream or whipped cream, if desired. Enjoy!
Notes
Peaches - Use your favorite variety of peaches for this recipe (any kind works). For reference, my 4 peaches weighed 1 pound 9 ounces (710 grams) total. It's fine if your peaches are slightly heavier or lighter though.Food Processor - I recommend using a food processor for the pastry, if you have one. It's faster and less messy. Pulse in the butter into the salt / flour mixture, then add the ice water, stopping when the dough starts to come together in a rough ball.Lemon - Use the leftover lemon juice to make a glass of lemonade or this simple lemon olive oil salad dressing.More Tips - Check out the info before this recipe card for helpful tips, step by step photos, substitutions, and more!