2To make the filling, bring half the blueberries and the rhubarb to a simmer in a medium saucepan along with the sugar and cornstarch. Cook until the fruit breaks down and the berries burst, and the sauce bubbles and thickens. Remove from the heat and stir in the remaining blueberries.
3On a lightly floured surface, roll each half of the thawed puff pastry into a 10-inch square, and cut each into 4 squares. Beat the egg lightly with a fork, and brush two edges of each square with it. Put a large spoonful of filling in the middle and fold over to enclose it, sealing with the tines of a fork, and using the egg wash as glue.
4Place all the triangles on a parchment-lined baking sheet (or two, if you need to), and brush the tops with the beaten egg. Bake for 15-20 minutes, until golden. Serve warm.