Preheat oven to 425 and prepare a pizza or baking pan by lining it with parchment paper or a silicone liner. A Pizza pan or large baking sheet sprayed with Pam or greased would also be fine.
For Pastry, put flour, sugar and salt in a bowl. Cut in the butter until mixture is crumbly. Add the cold water until it forms a dough. Knead the dough just until mixture comes together and is well blended and form it into a round ball.
Pat into a round disc on plastic wrap, fold over the plastic wrap and refrigerate while preparing the streusel filling.
For streusel filling, place the flour, sugar, cinnamon, nutmeg and salt in a bowl. Cut in butter until it's a crumbly "streusel" consistency and set aside in the refrigerator.
Peel the 4 apples and cut them into wedges or pieces about and inch thick.
Remove pastry from fridge and roll out on floured surface into a circle about 1/4 inch thick and approx. 11 to 12 inches in diameter). Transfer pastry to prepared baking sheet.
Remove streusel mixture from fridge and sprinkle about a tbsp. in the middle of the pastry. Place half of the cut-up apples into the middle on top of the streusel. Top with about half of the streusel remaining. Add the rest of the cut up apples (making sure to leave about 2 inches of pastry around the sides for folding). Top with remaining streusel and fold the edges over the apples pinching each of the edges together so that only the middle portion of the apples are still exposed. Brush pastry with a bit of milk and a sprinkling of sugar and place on middle rack in oven. After 5 minutes turn the oven down to 400 and continue baking for 20 to 25 minutes until golden brown.
Can be served hot or cold. Store any remaining crostata in a lightly covered container in cool area.