Preheat oven to 400° and line a baking sheet with parchment paper.
Roll puff pastry into a 12-in. square. Cut into 8 rectangles, 3 by 6 in. each.
Spread 1/4 tsp. mustard on half of each rectangle, leaving a 1/4-in. border at edges. For each, top mustard with 1/8 of cheese, then 1/8 of loosely bunched ham ribbons. Brush border with water. Fold empty dough half over ham and cheese and crimp edges with a fork to seal.
Put hand pies on prepared baking sheet. Cut a few slits in top of each pie. Bake until brown and crusty, 20 to 25 minutes. Cool on a rack, then chill in an airtight container. Bring to room temperature to serve.
Make ahead: Up to 2 days, chilled airtight.