Preheat oven to 375 °F.
Dice apricots, plums and peach or nectarine (with skin on) into pieces about ½-inch in size and toss with melted butter. Stir in brown sugar and cinnamon and set aside.
On a lightly floured surface, roll out puff pastry to a rectangle 12 by 8 inches. Cut into 6 squares. Spoon fruit filling into center of each square. Brush edges of each square with egg wash and bring points of pastry together to meet. Pinch edges together and place dumpling in a 6-cup baking dish. Brush tops of dumplings with egg wash and sprinkle generously with cinnamon sugar. Bake for 25 to 30 minutes, until pastry is a rich golden brown. Let cool for 10 minutes before serving.
Great with a scoop of vanilla ice cream.