Gently roll out the puff pastry on a piece of parchment paper. Move the rolled puff pastry to a large baking sheet using the parchment paper. Spread the jam over the top, leaving a ½-1 inch border around the edge.
Arrange the cut apples over the jam.
In a small bowl, stir the brown sugar, cinnamon, and nutmeg. In another small bowl, whisk the egg and water together. Sprinkle the brown sugar mixture over the apples and brush the edges of the puff pastry with the egg wash.
Dot the top with small butter cubes. Bake for 20-25 minutes until golden brown on the edges.
Notes
Store the apple tart in an airtight container in the refrigerator for up to 3 days. Reheat at 325 degrees for 10-15 minutes or until warm.