Place the four, sugar and butter in a bowl and place in the freezer for 15 minutes. Rub the butter into the flour until the mixture resembles breadcrumbs, or blend in a food processor.
Add the egg yolk, vanilla extract and enough ice cold water to form a dough. Press the dough into a flat disc, wrap in clingfilm and chill in the fridge for 30 minutes.
Heat the oven to 200ºC (180º fan) 400ºF, gas 6. Mix the rhubarb and strawberries together and place in a pie dish. Sprinkle over the sugar and orange juice.
Roll the pastry out and cut a circle to make the pie lid. Place on top of the pie, moistening the edge top make a good seal, and decorate with pastry offcuts. Brush with the beaten egg and bake for 30 - 40 minutes or until the pie is golden brown.