In a medium bowl, add first 3 ingredients and whisk to combine. Add butter and using fingers, press into flour until dough has a coarse, sandy texture. Sprinkle dough with half of the cold water. Using a rubber spatula, combine until a shaggy dough forms. Add more water as needed.
Turn dough out onto a floured surface. Form into 2 round discs. Wrap each disc in plastic wrap and place in fridge. Let dough chill for at least an hour.
Preheat oven to 425 degrees F. In a medium bowl, combine apple, brown sugar, flour, lemon juice, cinnamon, and nutmeg. Set aside.
Once dough is chilled, remove from plastic wrap and place on floured surface. Roll out dough to 12 inch circle and transfer to baking sheet. Repeat with the second dough.
Divide apple slices among the 2 crusts, leaving a 2 inch boarder. Arrange apples as desired. Fold the edges of the dough over the apples. Prepare an egg wash by whisking egg with milk. Brush over the crust edges.
Bake until apples are soft and crust is golden brown, about 35 minutes. While galettes cool, prepare the caramel sauce. In a sauce pan over medium heat, combine sugar and water. Continuously swirl the mixture until it reaches a deep amber color.
Turn off heat and remove from burner. Slowly stir in heavy cream until smooth. Return pan to warm burner and add bourbon. Continue to stir for 1 minute. Let cool completely. The sauce will thicken as it cools.
Serve galette with a scoop of vanilla ice cream and drizzle with caramel sauce. Enjoy!