Make a berry filling. Cut out 2 circles of puff pastry (any size); freeze for 15 minutes. Cut slits across surface of 1 piece. Spread berry filling onto remaining dough, leaving 1/2-inch border.
Place slit dough on top of filling; adhere to bottom piece of dough with beaten egg. Brush top with egg, and sprinkle with sanding sugar. Transfer to a rimmed baking sheet, and freeze for 15 minutes. Bake at 400 degrees until golden, about 15 minutes.