Prepare the Filling: Begin by caramelizing the onions. Warm the olive oil in a medium skillet over medium heat. Add the onions and prunes and cook, stirring occasionally, until golden brown. This step takes longer than you think (about 30 minutes or so), so be patient. Set aside to cool.
In a mixing bowl, combine the diced ham, shredded Gruyère cheese, caramelized onion mixture, Dijon mustard, honey, chopped rosemary, salt, and pepper. Mix until all ingredients are well incorporated.
Assemble the Hand Pies: Roll out one disk of the gluten-free pie dough on a surface that has been dusted with gluten-free flour to about 1/4 inch thickness. Use a 6-inch, round cookie cutter to create circles for the hand pies. Repeat with the remaining dough. Re-roll the dough as necessary. You'll get about 12 circles.
Spoon a heaping tablespoon of the ham, cheese, prune, and caramelized onion mixture onto one half of each pie crust circle, leaving a small border around the edges.
Fold the other half of the pie crust over the filling, forming a half-moon shape. Seal the edges by pressing them together with a fork.
Egg Wash and Garnish: Beat the egg and brush it over the tops of the hand pies for a golden finish. Optionally, sprinkle sesame seeds, poppy seeds, or everything seasoning for an extra touch.
Bake to Perfection: Place the hand pies on a parchment-lined baking sheet and bake in a preheated oven at 375°F (190°C) for 15-20 minutes or until the crust is golden brown.
Serve and Enjoy: Allow the hand pies to cool slightly before serving. These savory delights can be enjoyed warm, bringing a burst of flavor to your holiday celebrations.