Whisk flour, sugar and salt together in a mixing bowl.
Cut in the cold butter with your fingers or a pastry cutter until you have a mix of pea-size and flat, flaky pieces. Drizzle in ice water 1 Tbsp at a time, tossing, until the dough just holds together when squeezed. It won't be a smooth dough; this is normal.
Press into a disk, wrap, and chill at least 1 hour.
Make the Filling and Assemble
After the dough has chilled, gently toss peaches with sugar, cornstarch, and lemon juice. Let sit 10 minutes.
Roll out the dough. Lay out a piece of parchment paper. Lightly flour it, then place your dough on top. Cover with another piece of parchment paper. Then, roll the dough out between the two sheets into a 12-inch round. Peel off the top sheet and use the bottom sheet to lift the dough into the skillet, letting the edges drape up the sides.
Mound the fruit in the center, or you can create a little design with the fruit. Just leave a 2-inch border.
Fold the border up and over the fruit, pleating as you go. Lightly drizzle the fruit with the excess sugar mixture from the bowl. Make sure you don't drizzle any on the crust.
Brush the crust with egg wash and sprinkle with coarse sugar.
Chill and Bake
Return the galette to the fridge and chill for 15–20 minutes; while it chills, preheat the oven to 400°F. Once the crust is firm and the oven is hot, bake for 30 minutes, or until the crust is a deep golden color.
Cool in the skillet for at least 20 minutes before slicing so the filling sets.
Notes
Galette crust is very similar to homemade pie crust; you want it very cold before rolling, and you don't want to overwork the dough, which warms it up. Be sure to use icy cold water and follow the recommended chill times.
Don't skip the cornstarch! It's essential for turning the peach juice into a sticky syrup, so you don't end up with a soggy crust on the bottom of your galette.
This galette is delicious on its own, or you can serve it with a scoop of vanilla ice cream!