Spread the pie crusts out onto a flat, very lightly floured surface. Using a 4" circular cup or bowl, cut 4 circles out of each crust. Roll the scraps into a ball, and then use a rolling pin to flatten them out into another two pie crusts. Cut another 2 circles out of each.
Spread parchment paper out onto a large baking sheet. Place half the pie crust circles evenly out over the paper.
Spread 1 tbsp of Kaukauna® Sharp Cheddar Spreadable Cheese evenly out in the center of the circle, leaving a 1/4" boundary clean around the perimeter.
In the bowl of a stand mixer, whip together the cream cheese, horseradish, and mustard until the mixtures smooth and evenly incorporated.
Place a loosely folded piece of roast beef in the center or each prepared circle, on top of the cheese, and maintaining the clean edge.
Add 2-3 spinach leaves (if using). Top with 1 tablespoon of the cream cheese mixture and lightly spread.
Using your fingers, gently squeeze the outer edges of the remaining pie crust to spread them out slightly. Place them evenly over top the prepared pies. Using a fork, press gently to close the edges all the way around until the hand pies are all completely sealed.
Crack the egg into a small bowl. Add the water, and beat until smooth. Using a pastry brush, lightly brush the top of each pie with the egg wash.
Bake the handpies at 400 degrees for 12-15 minutes, just until cooked and golden brown/flaky.
Serve the handpies immediately, or refrigerate them (covered) for up to 48 hours, reheating just before serving.