Make the pastry: In a medium-sized bowl, whisk together flour, sugar and salt. Using a pastry cutter or two knives, cut butter into flour mixture until it becomes loose and crumbly with pea-sized butter pieces distributed throughout. Drizzle cold water over and mix until the water is evenly distributed. If the dough seems dry, add another tablespoon of water. Turn dough out onto a floured surface and, using your hands, fold the dough into itself gathering any loose pieces and surface flour as you go. Repeat two or three more times until the dough holds together in a ball. Wrap in plastic wrap and let chill in the refrigerator for 1 hour or up to 3 days if you don’t plan to bake it right away.
While the dough is chilling, assemble apple filling: In a large bowl, toss together sliced apples, brown sugar, lemon juice, maple syrup, cinnamon and nutmeg. Cover and set aside in the refrigerator until you’re ready to bake.
Preheat oven to 400ºF (200ºC) and prepare a large parchment-lined baking sheet. In a small bowl, prepare egg wash by whisking together egg and milk and set aside.
Remove dough from refrigerator and transfer to a floured surface. Roll out dough into a circular shape measuring roughly 12” in size. If the dough is sticking, lift the edges and add more flour to the surface. To facilitate transferring, roll the dough around your rolling pin then open it up across the parchment. Transfer apple filling to the centre of the dough leaving a 2” border around the edges. Fold the edges up and over the apple slices, then brush crust with egg wash and sprinkle coarse sugar over the edges.
Bake for 35 minutes until the crust is nicely browned and the filling is bubbling. Let cool, top with fresh thyme leaves if desired and serve!