Cream butter into flour, sugar, and salt until butter is turned into pea sized pieces. Add water and mix until just incorporated. Separate into 4 balls and roll out into thin circles. Place in fridge and chill. Mix together the 1/2 cup brown sugar, cinnamon, and nutmeg. Set aside. Peel, core, and cut 4 large apples and cover with spice mixture. Top each apple with a tsp of butter. Place each cut apple in centre of cooled pastry and bring edges of pastry over the apple to completely cover it. Make egg wash by beating the egg and adding the milk. Cover top with egg wash and sprinkle with sugar. Bake at 375°F for 40 minutes or until golden brown.