Preheat the oven to 425 degrees . On a lightly floured surface, cut the dough into 4 rectangles, pressing the perforated seams together to seal. Roll out each rectangle to form a 6-inch square and transfer to a parchment-lined baking sheet; refrigerate while you make the filling. In a small bowl, whisk the egg with 1 tsp. water.
In a large skillet, melt the butter over low. Add the leeks; season with salt and pepper. Cover and cook until the leeks soften, about 7 minutes. Off heat, stir in the ham, garlic, rosemary and mustard; season.
Dot the cream cheese on the dough squares, leaving a 1-inch border; top with the leek mixture. Fold the points of each square into the center, pinching the tips together to seal. Brush the pastry with the egg. Bake until puffed and golden, 10 to 12 minutes.