Melt the butter in a large skillet over medium heat. Add the onions with a good pinch of salt and a few turns of pepper and cook, stirring, until very soft, about 10 minutes. Reduce the heat to medium low, add 1/4 cup water and cook, stirring occasionally, until the water is evaporated and the onions are lightly caramelized, 25 to 30 minutes.
Preheat the oven to 425 degrees F. Line 2 baking sheets with parchment paper and set aside.
Divide the pie dough into 8 equal parts and shape into balls (keep half of the dough balls covered in the fridge while you aren't working with them to keep them cold). On a lightly floured surface, roll out the balls to ovals that are 8 inches long and about 6 inches wide. Top the bottom half of each dough piece with a pile of cheese and a pile of onions, leaving a 1-inch border. In the top half of each dough piece, punch out an upside down ghost face with piping tips or tiny round cookie cutters (it's upside down so that when you fold it over on top of the filling, it's right-side up). Brush the edges with egg, fold the top down over the filling and fold the sides in on themselves so that you have ghost shape. (Or, rather, a shape of a tiny kid in a bedsheet ghost costume that's lying down.) Press around the side and bottom edges to seal, transfer to the baking sheets, brush the tops all over with egg wash and sprinkle with a little salt.
Bake until golden brown; begin checking for doneness at 20 minutes. Let cool slightly.
Mix together the Dijon mustard and honey in a small bowl. Serve the hand pies warm or at room temperature with the mustard sauce.
To make the dough, combine the cider vinegar and 6 tablespoons water in a measuring cup and stick it in the fridge (or the freezer even) to get really cold. In a large bowl or food processor, combine the flour, sugar and salt. Add the butter and either use your hands to toss it with the flour and pinch the butter into flat sheets, or pulse in the food processor, incorporating the butter so that about 75 percent of the mixture is mealy. The rest of the mixture should have some slightly larger, pea-sized bits of butter. Drizzle in the vinegar and water and mix with your hands or continue to pulse in the food processor just until the mixture comes together to form a dough. If it seems dry or is having a hard time coming together, add a bit more water a few drops at a time until it comes together. Turn it out onto a clean surface, using your hands to press on any stray crumbs, and divide the dough in half. Pat the halves into discs, wrap with plastic wrap and refrigerate for 30 minutes.