I always make short pastry myself and according to 3-2-1 rule:
3 parts flour
2 parts butter
1 part sugar
For the medium size and rather thin pie I used:
3oo gram flour
200 gram cold butter
100 gram brown sugar (better caster sugar but I didn’t have)
300 gram rhubarb (peeled and sliced)
1 teaspoon baking powder
pinch of salt
2 tablespoons white sugar
Mix flour with baking powder, salt and 100 gr sugar. Add 1 egg and 1 egg yolk (keep the egg white for later). Chop in cold butter and use a wide-blade knife to chop until very small crumbles of butter are created. The point is to cut the kneading time to minimum. Knead only as much as necessary for the dough not to be falling apart and move to the fridge for an hour.
In the meantime cut the rhubarb and add 2 tablespoons of sugar and set aside for 1 hour – the rhubarb slices will release juices by that time.
Take the dough out of the fridge and divide into 2 parts 2/3rd for the bottom and 1/3rd for the top.
Roll out the dough (not too hard, it’s even better to use just hands to place small chunks on the baking tin) and place in the baking tin on, a baking paper. Fill the tin with the rhubarb and blackberries. Add some 1 tablespoon of sugar, if the fruits are too sour. Roll out the remaining dough and cover the tin. Beat slightly the egg white and brush it over the top of the pie. Bake for 20-30 mins in 180 C degrees.