Prepare your workspace and ingredients. Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Allow refrigerated dough to come to room temperature for about 15 minutes.
Prepare the apples properly. Core, peel, and slice apples into thin, uniform slices about 1/8-inch thick. Toss them immediately with lemon juice in a large bowl to prevent browning.
Create the filling mixture. In a small bowl, combine brown sugar, flour, cinnamon, nutmeg, and salt. Sprinkle this mixture over the apples and gently toss until evenly coated, being careful not to break the slices.
Roll out your dough carefully. On a lightly floured surface, roll the pastry dough into a 12-inch circle about 1/8-inch thick. Transfer to your parchment-lined baking sheet.
Arrange apple slices decoratively. Place the seasoned apple slices in the center of the dough, leaving a 2-inch border around the edges. You can create concentric circles or simply pile them in a rustic mound.
Fold the edges with confidence. Gently fold the dough border over the apples, pleating as you go around. The center will remain open, showcasing the fruit.
Add butter for richness. Dot the exposed apple filling with small pieces of cold butter – this will create pockets of richness as it bakes.
Apply the egg wash thoroughly. Brush the folded dough border with beaten egg and sprinkle with turbinado sugar for a beautiful golden finish and sweet crunch.
Bake until perfectly golden. Place in the preheated oven and bake for 40-45 minutes, or until the crust is golden brown and the apples are tender when pierced with a knife.
Cool before serving. Allow the galette to cool for at least 15 minutes before slicing. If using, warm the apricot jam slightly and brush over the apples for a glossy finish.