Preheat oven to 425 degrees. Line a baking sheet with parchment paper and set aside.
On a lightly floured surface, roll out puff pastry sheets to approximately 12 inch squares. Cut each sheet into four squares.
Spread ½ teaspoon of mustard onto each square, avoiding ½ an inch around the edges. Spoon 2 ounces of ham and 1 tablespoon of shredded cheese onto a corner of each square. Fold the opposite corner of each puff pastry over to cover filling. This will form a triangle.
Press the edges with a fork to seal. Cut a small vent into the tops of each turnover (or prick with a fork).
In a small bowl, add egg, 1 tablespoon of water, and garlic powder. Whisk together to create an egg wash. Brush each turnover with egg wash.
Transfer turnovers to parchment lined baking sheet. Bake for 20-25 minutes, or until turnovers are golden brown.