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 using a hand mixer, reserve a tablespoon of beaten egg for brushing later, then 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.
Allow the dough to rest for 15 minutes.
Divide the dough in half, and roll out 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, you will need 16 strips.
To lattice the pie, arrange 8 strips of dough over the filling vertically - the longer strips will go in the middle, and the shorter ones towards the edges.
Starting from the right hand side, fold over strip no 1, 3, 5 and 7.
Lay one unused strip perpendicularly on top, and place the folded strips back into their place.
Next, fold over strips no 2, 4 and 6 and 8, then arrange another dough strip perpendicularly, and fold the strips over again.
Repeat this process until all the dough strips have been used and you have a nice lattice top.
Bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 40 minutes.
Leave to cool, then slice.