Roll out the pastry following the instructions and line twenty 8 cm (3¼ inch) round fluted loose-based tart tins. Cut out twenty circles with a 9 cm (3½ inch) diameter to use for the lids. Set the pastry cases and lids in the freezer for at least 20 minutes. Preheat the oven to 200°C (400°F/Gas 6).
Put the pears in a bowl and add the butter, sugar, cinnamon and lemon juice, tossing well to combine. Place them into a baking dish that holds them snugly in a layer no more than 2 cm (¾ inch) deep. Cook in the oven for 12 minutes, or until softened but still holding their shape. Remove from the oven, transfer to a bowl and add the blueberries while the pears are still warm, mixing well. Allow to cool to room temperature, then drain in a colander.
Spoon 2½ tablespoons of the fruit mixture into each tart shell, piling it slightly higher than the top of the shell.
To attach the lids, brush the rim of the pastry base and the lid with a little egg wash and lay the lid over the base. Squeeze gently between your thumb and index finger to make a good seal. Brush the tops with egg wash and make a small hole in the middle to allow steam to escape. Sprinkle a little caster sugar on top and place the pies on baking trays. Reduce the oven temperature to 180°C (350°F/Gas 4) and cook the pies for 20–25 minutes, or until the pastry is a deep golden colour. These pies are best served hot but can also be eaten cold.