Macerating the fruit in a bit of sugar and acid (strawberries and balsamic vinegar are natural allies) helps draw out their natural juices and makes an ideal filling for pastry shells.
Preheat oven to 400 degrees. Unfold each sheet of puff pastry onto a parchment-lined baking sheet and cut each into 4 squares. With a paring knife, lightly score a border 3/4 inch from edges. Brush border with egg and sprinkle with sugar. Freeze 10 minutes. Bake until puffed and golden, about 15 minutes. Let cool. Gently press center to flatten.
Meanwhile, combine fruit, sugar, citrus juice, and salt. Let stand 10 minutes. Fill shells with cream and top with fruit mixture. Serve immediately.