Begin by peeling and dicing your potatoes into 1/2-inch cubes. Place them in a pot of salted water and bring to a boil, cooking until tender, about 10-12 minutes. Once cooked, drain and let them cool slightly before moving on to the next step.

While your potatoes cool, finely dice the onions and melt a tablespoon of butter in a skillet over medium-low heat. Sauté the onions until they become tender and translucent, about 5-7 minutes, being careful not to brown them. Once cooked, remove from heat and allow to cool.

In a large mixing bowl, whisk together the heavy cream, Dijon mustard, all-purpose flour, kosher salt, and black pepper until smooth. In another bowl, mix your cooled potatoes, sautéed onions, and sharp cheddar cheese. Pour the cream mixture over the potato blend and gently stir until evenly combined.

On a lightly floured surface, roll out the chilled puff pastry sheets to about 1/8-inch thickness. Use a sharp knife or pastry cutter to cut the dough into 4x6 inch rectangles, keeping some extra pastry for the tops of your hand pies. Try to work quickly to keep the pastry cold.

Take one rectangle of pastry and brush the edges with egg wash to help seal. Place a generous spoonful of the Cheese Onion and Potato filling in the center, then cover with a second pastry rectangle. Press the edges firmly and crimp them with a fork to ensure no filling escapes during baking.

Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper and arrange the filled hand pies on it, leaving some space between each one. Brush the tops with egg wash, cut small slits for steam to escape, and bake for 25 to 30 minutes until they are beautifully golden brown and flaky.