By Hand Method:
Mix flours and salt in a large bowl. Chop butter into 1/2-inch chunks and add to flour mixture. Pinch or cut butter into flour using your fingers or a pastry cutter, breaking it into pieces about the size of large peas. If using your hands, work quickly to keep butter from melting.
Pour water into flour mixture. Mix dough with your hands or about 10 to 15 turns of a wooden spoon, until it just starts to come together. The dough will be very tough and should remain in a few large chunks.
Place dough on a lightly floured surface and push chunks together. Knead 4 to 5 times, just until it holds together. Flatten dough into a disk about 1 inch thick. Wrap tightly in parchment paper and let rest in the refrigerator for at least 1 hour.
Food Processor Method
Pulse flours and salt in the bowl of a food processor 2 or 3 times to combine. Chop butter into 1/2-inch cubes and add to flour mixture. Pulse for 1 second about 8 times, until butter is in pieces about the size of large peas. Add water and pulse 3 to 4 times, until dough begins to come together. It may remain in a few large chunks.
Place dough on a lightly floured surface and push chunks together. Knead 4 to 5 times, until it holds its shape. Flatten dough into a disk about 1 inch thick. Wrap tightly in parchment paper and let rest in the refrigerator for at least 1 hour.
Preheat the oven to 375 degrees F.
Combine apples, sugar, lemon juice, cinnamon, and nutmeg.
Divide the dough into two halves and roll one out into a rough circle. Move it to a baking sheet.
Heap the prepared apples into the center and gently fold the edges of the dough around to encircle the apples.
Brush the dough with the milk.
Bake the crostata for 35 to 45 minutes, until the top of the crust is golden brown, and the apples are tender.