In a medium sized bowl, mix the corn starch and sugar and toss the blueberries in this. Then add the lemon juice to coat it all and set aside.
For the topping:
In a small bowl, mix the flour, brown sugar, salt and ground cinnamon. Add in the cold butter cubes, and using two forks, , without ever using your hands (this could also easily be done using a food processor in just a few seconds).
Pour the blueberries in a round 9 inches (22cm) diameter, or a rectangular or square pan of similar dimensions) and fully cover them with the flour mixture.
Bake at 375°F (180°C) for 15 to 25 minutes or until the top starts to turn golden.