1) Mix the flour, sugar, and salt in a processor. Add the butter. Pulse until the mixture resembles coarse breadcrumbs. Add the iced water and pulse until moist clumps form. Gather the dough into a ball; flatten into a disk. (If the dough still crumbles and does not form into a ball, add another tbsp of iced water.) Wrap the dough in clingfilm and refrigerate until firm, about 1 hr.
For the filling:
1) Position the rack in the centre of the oven and preheat the oven to 200°C/ gas mark 6.
2) Combine the apples, 50g sugar, and lemon juice in a large bowl; toss gently to blend. Set aside for 10 mins.
3) Meanwhile, dust a large sheet of parchment paper with flour and roll out the dough on the paper to a 28cm round. Transfer the dough on the parchment paper to a heavy large baking sheet. Spoon the apple mixture over the dough, leaving a 5cm border. Fold the dough border over the filling to form an 20cm round, leaving the apples exposed in the centre.
4) Pleat loosely and pinch the dough to seal any cracks. Brush the crust with the egg wash and sprinkle with the remaining 15g sugar.
5) Bake the crostata until the crust is golden and the apples are tender, about 40 mins. Transfer the baking sheet to a rack; cool for 10 mins. Slide a metal spatula under the crust to free the crostata from the baking sheet. Cool the crostata to lukewarm. Sprinkle with the almonds and serve.