Make the crust: In a large bowl, combine the flour, sugar, and salt. Add the cold butter cubes and use a pastry cutter or your fingers to incorporate the butter until the mixture resembles coarse crumbs. Gradually add the ice water, one tablespoon at a time, stirring with a fork until the dough begins to come together. Turn the dough out onto a lightly floured surface, shape it into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

1. Prepare the apple filling: Preheat your oven to 375°F (190°C). Peel, core, and slice the apples. In a large bowl, toss the apples with lemon juice, granulated sugar, brown sugar, cinnamon, nutmeg, salt, and flour. Set aside to let the flavors meld.
2. Make the crumb topping: In a small bowl, combine the flour, granulated sugar, brown sugar, cinnamon, and salt. Add the cold cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture forms coarse crumbs. Set aside.
3. Assemble the galette: Roll out the chilled dough on a floured surface into a rough 12-inch circle. Transfer it to a parchment-lined baking sheet. Spoon the apple mixture into the center of the dough, leaving about a 2-inch border. Fold the edges of the dough over the apples, pleating as you go to form a rustic edge.
4. Add the crumb topping: Sprinkle the crumb topping evenly over the apples.
5. Bake: Bake the galette for 40-45 minutes, or until the crust is golden brown and the apples are tender. If the edges of the crust brown too quickly, cover them with aluminum foil.
6. Serve: Let the galette cool for at least 15 minutes before slicing. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.