Preheat the oven to 350 degrees. Roll out the puff pastry on a well-floured board. Make slits in the puff pastry like pictured above. Put the puff pastry on a baking sheet lined with parchment paper.
Squeeze the mustard down the middle of the puff pastry.
Spread the mustard out using a pastry brush.
Arrange the Gouda slices evenly on top of the mustard.
Overlap the strips of puff pastry.
Whisk together the egg and a few teaspoons of water.
Brush the egg wash over the braided puff pastry.
Seal the ends.
Bake in the oven for 20 to 25 minutes or until puffed and golden brown.
Serve with sliced apples.