Peel and cube Yukon Gold potatoes, boil in salted water for 15-20 minutes until tender, then drain and cool.

Melt butter in a skillet, sauté diced onions for 5-7 minutes until translucent, then cool.

In a mixing bowl, whisk together heavy cream, Dijon mustard, flour, salt, and pepper. Fold in potatoes, onions, and cheese.

Roll out thawed puff pastry on a floured surface, cut into 4x6 inch rectangles.

Preheat oven to 425°F (220°C). Brush edges of one rectangle with egg, add filling, place another rectangle on top, and crimp edges.

Transfer to a baking sheet, brush with egg wash, cut slits for steam, and bake for 25-30 minutes until golden brown.