For filling: Place peeled and sliced apples in lemon juice and stir to coat. Set aside. Combine sugar, cornstarch, cinnamon, salt, nutmeg and water in a saucepan. Bring to a boil. Boil for 2 minutes, stirring constantly. Add apples and return to a boil. Reduce heat. Cover and simmer until apples are tender, about 6 minutes. Set aside to cool.
To assemble: Make egg wash by whisking egg yolk and 2 Tbsp. water with a fork. With a rolling pin, roll out pastry halves to 1/8". Cut each half into 4-5 rectangles. Place 1-2 Tbsp. of apple filling on half of one side of each pastry rectangle. Sprinkle about 1-2 Tbsp. crushed Frosted Flakes over top of apple filling. Brush the edges of the pastry with egg wash and fold pastry in half, sealing in the apples. Press the edges together then crimp with a fork. Place on cookie sheet(s) covered with parchment paper.
Chill in refrigerator for 30 minutes. After 30 minutes, preheat oven to 375º and pull out cookie sheet(s) from fridge. Brush tops with egg wash. Slice about three vents in the top of each pie to allow steam to escape. Bake for 20-25 minutes or until golden and the pastry is cooked though. Makes 8-10 hand pies. Can cut recipe in half to make 4-5 hand pies if desired. Enjoy!