Take the pie crust out of the fridge and allow it to sit at room temperature for about 10 minutes. This will make it easier to roll out (If you made your own pie crust using my recipe, you will only use half of the dough from the recipe).
Cut the crust into thirds and form 3 balls. Roll out each ball on a lightly floured surface with a rolling pin to be 8-9" in diameter. You don't need a perfect circle since you will be folding up the edges.
Place the rolled-out circles onto baking sheets lined with parchment paper. Place the baking sheets into the fridge while you prepare the filling. This helps create a flaky crust!
Preheat the oven to 400°F and place the oven rack in one of the lower settings in the oven.
Core and cut the apples into slices, then cut each slice in half again. The smaller pieces are easier to fit into a mini galette. Place the apple slices into a large bowl. Combine ¼ cup brown sugar and the flour, lemon zest, lemon juice, cinnamon, and nutmeg with the apples.
Remove the chilled crusts from the fridge and spoon the apple mixture on top of each one starting at the middle. Leave about 2" around the edges. Fold the edges up and around the apples on each galette, creasing where needed.
Beat the egg and brush the crust edges with it. Mix together 1 tablespoon sugar and 1 tablespoon brown sugar and sprinkle on the edges on top of the egg wash.
Place the galettes in the oven and bake for 30 minutes. Allow the galettes to cool for a few minutes. Top with the salted caramel sauce and vanilla ice cream if desired. Enjoy!