Whisk the 1 ¾ cups gluten free flour, 2 tablespoons granulated sugar, and ¼ teaspoon fine sea salt in a medium bowl. Then work in the ½ cup unsalted butter using a fork or your hands. Drizzle the ¼ cup ice water over the mixture, adding more if needed to form a smooth dough. Knead it a couple of times, then form a ball, flatten it into a disc, wrap it in plastic, and place it in the fridge for 1 hour.
Make Filling
In a large bowl, toss together the sliced 4 medium apples, ½ cup granulated sugar or brown sugar, 1 teaspoon ground cinnamon, and ¼ teaspoon ground nutmeg together in a large bowl. Let it sit until the dough is ready.
Have a large baking sheet ready. Place a piece of parchment paper or a silicone baking mat on the counter.
Lightly flour the parchment, then take the dough out of the fridge, place it on the parchment and roll it into about a 12-inch circle
Arrange the apples (and any juices from the bottom of the bowl) into the center of the dough, leaving a 2–3-inch border all around them. Then gently fold and crimp the edges of the dough over the filling, overlapping the dough as necessary. Gluten free dough is a little tricky, so go slow. Press to seal the edges and brush with the egg wash if using.
Place the galette in the fridge for 20 minutes or up to 8 hours (covered).
Preheat oven to 400°F.
Take the galette on the baking sheet out of the fridge and bake until the filling is bubbly and the crust is golden brown and set, about 35–45 minutes. Check at 35 minutes and keep checking if it needs a little longer.
Remove the galette from the oven and it allow to cool on the baking sheet for 10 minutes before slicing and serving.
Notes
Chill the assembled galette.This may seem like just another step, but if you let the formed galette rest in the fridge before baking, it will keep the butter nice and cold, which will prevent the crust from spreading out too much in the oven. This is especially important when working with a gluten-free dough, which is more delicate.
Rest the dough before assembling. Any pie crust will have you rest the dough for at least 30 minutes before using, and it's especially important when using gluten-free flour so that the flour hydrates while the butter chills.
Roll the dough between parchment. Dust the bottom piece of parchment with a bit of gluten-free flour, top it with another piece of parchment, then roll out the dough with a (gluten free) floured rolling pin. This will help keep the dough from sticking.
Temperature matters. If it's quite hot in your kitchen, roll out the dough as quickly as you can, then assemble the galette. The warmer the kitchen, the stickier the dough will become.
Add lemon zest. Sprinkle in the zest of 1 lemon to the filling for a bit of citrus brightness.
Don't overstuff the galette. Tempting as it may be to go wild on the apples, if the galette is too full, it may fall apart when baked. If you feel like you have too much filling, set some aside (make a quick applesauce or just snack on the extras) so the galette isn't overly full.
Rest after baking. Let the galette cool fully to room temperature before slicing it to allow the crust to set.
Sprinkle with sugar. Scatter coarse sugar over the top of the assembled galette for extra sweetness.