Peel and dice the potatoes into ½-inch cubes, place them in salted boiling water, and cook until tender, about 10–15 minutes.

In a skillet, melt butter over medium-low heat and add the diced onions. Sauté until soft and translucent, about 5–7 minutes.

In a mixing bowl, combine the cooled potatoes, sautéed onions, cheddar cheese, heavy cream, Dijon mustard, flour, salt, and pepper. Mix gently and set aside.

Whisk egg and a splash of water together to create the egg wash for later use.

Roll out the puff pastry and cut it into rectangles. Brush edges with egg wash, spoon filling into center, fold over and crimp edges to seal.

Preheat oven to 425°F (220°C). Arrange hand pies on parchment-lined baking sheets, brush tops with egg wash, and cut slits for steam. Bake for 25–30 minutes until golden.