Begin by peeling and dicing your Yukon Gold potatoes, ensuring they’re cut into uniform pieces for even cooking. Place them in a large pot of salted boiling water and cook for about 15–20 minutes until tender. Drain and cool.

While potatoes cool, heat a skillet over medium and add 2 tablespoons of butter. Once melted, sauté diced onions with a pinch of salt for about 5–7 minutes until tender and translucent.

In a mixing bowl, combine the cooled potatoes and sautéed onions. Add shredded cheddar, cream, mustard, and flour, seasoning with salt and pepper. Mix until well incorporated.

For the egg wash, crack an egg into a small bowl, whisk with a splash of water, and set aside.

Unroll thawed puff pastry on a lightly floured surface and cut into rectangles, approximately 4x6 inches. Brush edges with egg wash, add a spoonful of filling, fold over and crimp edges tightly.

Preheat oven to 425°F (220°C), line baking sheets with parchment, arrange assembled pies, brush tops with egg wash, and cut slits to allow steam to escape. Bake for 25–30 minutes until golden brown.

Let the pies cool on a wire rack for about 5 minutes before serving warm.