Make the dough: Mix the flour, sugar and salt in a medium bowl. Add the butter and cut it in with two forks or a pastry cutter until ½ cm crumbs appear. You can also use the food processor, just pulse a few times. Add the water and stir until the crust comes together. If the dough is too dry, add 1 tbsp water. Gently knead the dough a couple of times on a lightly floured surface. Shape the crust into a disc, cover it with plastic wrap and place it in the refrigerator for at least 1 hour and up to 3 days.
Make the filling as the dough chills: Mix the brown sugar, flour, lemon juice, cinnamon, nutmeg and apple slices in a large bowl. Cover tightly with plastic wrap and let sit in the refrigerator until the crust is ready.
Make the bourbon caramel sauce: Bring the heavy cream and unsalted butter to room temperature. Gently stir the sugar and water in a small saucepan. Place over a medium-high heat and don't stir or touch the pan until the sugar has caramelised. The color of the mixture will change gradually. It will start transparent, then a golden color and lastly a dark amber. When it starts changing colors, it will darken quickly. It will take about 10 minutes. Turn off the heat and remove the saucepan from the heat. Right away, slowly and carefully pour heavy cream into the sugar mixture, whisking vigorously to incorporate the cream. The mixture will bubble up. Lastly, mix in the butter until combined. Add vanilla extract, salt and bourbon and whisk until combined.
Preheat oven to 190°C. Line a large baking sheet with parchment paper.
On a lighly floured work surface, roll the dough into a 30 cm circle and transfer to the baking sheet.
Place the apple slices on the dough, leaving a 2 cm border all around. Don't add the juices accumulated in the bowl. Fold the edges of the crust over the apples, gently pressing the dough to stick. Drizzle 3 tbsp bourbon caramel sauce on the apples, but not on the crust.
Make the egg wash: beat the egg with 1 tbsp milk. Brush the crust edges with egg wash and sprinkle with 1-2 tbsp brown sugar.
Bake for 35-40 minutes or until the crust is golden-brown and the filling is bubbling. Cool down for 10 minutes, drizzle more bourbon caramel sauce if desired, slice and serve with vanilla bean ice cream.