Peel and finely dice the potatoes into roughly 1/2-inch cubes. Add the potatoes to a saucepan of water and place over high heat. Add a generous pinch of kosher salt and bring to a boil. Cook until the potatoes are tender. Drain and allow the potatoes to cool down before adding to the filling. You can spread the potatoes onto a plate and refrigerate to speed up the cooling process.
Meanwhile, peel and dice the onions. Place a large non-stick skillet over medium low heat. Add the butter along with the onions and a generous pinch of kosher salt. Sauté the onions just until they are tender to the bite but before they begin to brown. Cool the onions before adding to the filling. You can spread the onions onto a plate and refrigerate to speed up the cooling process.
In a small bowl, combine the heavy cream, Dijon mustard, flour, kosher salt, and pepper. Whisk together until smooth and there are no lumps. In a mixing bowl, combine the cooled potatoes, onions, and shredded cheese. Pour in the cream mixture and stir until well mixed.
In a small bowl, whisk together the egg and a splash of water to make an egg wash. Take 1 sheet of puff pastry and unroll onto a lightly floured counter. Lightly dust the puff pastry with flour and gently roll it into a square slightly bigger than the preformed sheet. Cut the sheet into six rectangles (3 pieces x 2 pieces).
Brush the edges of each rectangle with the egg wash and top with some of the filling (about 1/12th of the filling mixture). Spread the filling out onto the puff pastry making sure to leave a small border on all sides. T
ake another sheet of puff pastry and gently roll it out to match the first sheet. Cut into six rectangles (3 pieces x 2 pieces). Cover each rectangle with a piece of pastry and press the edges together. Use a fork to crimp the edges of each pie. Repeat with the remaining 2 sheets of puff pastry.
Preheat the oven to 425°F. Line two baking sheets with parchment paper. Arrange the hand pies on the baking sheets, fitting six per sheet if possible while still leaving a little room between each pie. Brush the tops of the pies with the egg wash. Use a sharp knife to cut 3 slits in top of each pie.
Bake the pies for 25 to 30 minutes or until the pastry is golden brown and the filling is bubbling. If you're baking two baking sheets at one time, rotate the pans after 15 minutes to prevent the pies on the bottom baking sheet from getting too dark. Let cool briefly before serving. Enjoy!