Preheat oven to 355°F (180°C) and grease your pie form (or use baking paper).
Pie crust
Add the flour, sugar and butter to a mixing bowl and mix with a fork or pastry cutter until mealy. Alternatively you can mix it in a food processor.
Add the water and press the dough together with your hands until it forms a solid ball.
Press the dough evenly across the bottom and up the edges of your pie dish.
Rhubarb filling
Rinse the rhubarb and cut it into small pieces.
Mix the sugar and corn starch together in your mixing bowl and then add the rhubarb, stirring until the mixture coats the rhubarb nicely. Pour this rhubarb mixture onto the pie crust.
Crumble topping
Mix the flour, sugars, and vanilla in your mixing bowl. Melt the butter and mix it in with the dry ingredients until you have a wet dough.
Use your hands to distribute the dough evenly in crumbles on top of your rhubarb filling.
Bake the rhubarb pie on the bottom shelf of your oven for about 1 hour and 10 minutes until cooked through. Enjoy!