To make the crumble, use an electric mixer to beat the soften butter with the sugar until creamy.
Add the flour and lemon zest, and use your hands to make a mixture that resembles crumbles.
Add half of this mixture to a pie tin, and press down and up the tin walls well to create a crust.
Cover the crust with non-stick baking paper, and fill it up with dry pulses or ceramic baking beans.
Bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 15 minutes, then remove the paper and pulses and continue to bake for a further 5 minutes until golden.
To make the filling, mix the blueberries with sugar, cornflour and lemon juice.
Transfer the filling onto the baked crust, and top with the remaining crumble mixture.
Bake for a further 15 minutes until the top is golden.
Cool down completely before slicing.