On a lightly floured surface, roll the puff pastry dough out 1/4" thick. Cut into rounds whatever size you like using a cookie cutter or glass rim. Place them on a parchment-lined baking sheet.
In a large bowl, toss together the rhubarb and strawberries. In a small bowl, stir together the sugar and cornstarch until no lumps remain. Add to the strawberry-rhubarb mixture and toss to coat well.
Spread some of the mixture over each pastry circle, leaving about an inch around the edges. Fold them over the filling, wherever they naturally fold. If you like, brush the edges with beaten egg and sprinkle with sugar.
Bake for 30 minutes, or until golden and the fruit is tender.