In a food processor, pulse together the flour, powdered sugar, and salt. Then pulse in the butter until the mixture becomes pea-sized crumbles.
Add the cold water and egg to the processor and pulse until just incorporated
Turn out onto a clean surface and knead the mixture just until the dough forms and no crumbles remain. Form the dough into a disc, wrap in plastic wrap, and refrigerate for 30 minutes.
Roll out the dough to just under 1/4 inch thick. Cut out 24 rectangles (or any shape). Brush the edges of the rectangles with eggwash, and spread honey mustard in the center. Layer on turkey, brie, and apple slices. Cover with another rectangle of dough, crimp the edges with a fork, and slash the top to prevent steam from gathering.
Bake on a parchment-lined sheet pan at 400°F for 28 minutes.