Line a baking sheet with parchment paper or aluminum foil. Preheat oven to 400 degrees.
On a lightly floured surface, unroll the puff pastry and using a rolling pin, lightly roll to seal seams and enlarge the dough just an inch or two.
Using a knife, divide the dough in half width wise.
Using a butter knife or offset spatula, spread each rectangle evenly with softened butter, mustard and onion powder.
Working with one rectangle of dough at a time, cut triangles about 3 inches wide at the longest end.
Pile about 1 tablespoon of ham in the center of the wide end of the triangle. Top with a 2x2 inch square of cheese.
Roll up pastry around the ham and cheese, tucking in the sides to cover the ham and cheese.
Repeat with the other roll ups.
Place on baking sheet and brush with egg and water mixture. Sprinkle with salt and pepper if desired.
Bake at 400 until golden and brown--about 15 minutes or so. Makes 10-12 roll ups depending on size.