To make the filling, add the chopped rhubarb and strawberries to a bowl, add the white and light brown sugar together with the cornflour, and mix well.
Set aside until you make the dough.
To make the dough, beat the egg with a pinch of salt, add the sugar, and continue to beat until you get a smooth, pale yellow consistency.
Add the milk, oil, baking powder and vanilla extract, and mix well.
Add the flour, and mix to get a smooth dough that does not stick to the hands; add more flour if needed.
Divide the dough in half, and roll the first half.
Grease and flour a pie tin or dish, and arrange the rolled dough.
Add the rhubarb and strawberries, making sure the juices are left in the bowl, otherwise the pie will be soggy.
Roll the second half of the dough, and cut into strips.
Arrange the strips on top of the pie, then beat the egg lightly and brush the top of the pie.
Bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 20-25 minutes.
If the strawberries and rhubarb are not soft enough at this stage, cover the pie loosely with a kitchen foil, and bake for a further 15-20 minutes, the foil will prevent the crust from browning too quickly.
Leave to cool, then slice.