Preheat the oven to 200°C / 400°F / Gas Mark 6. Grease a pie dish with a little butter
In a large bowl, combine the butter with the sugar until creamy. Add the flour and give it a good mix. It should start to look like coarse meal.
Slowly add half of the water and mix. Keep adding water until the dough comes together.
Cover the bowl with cling film and place in the fridge for about 60 minutes.
In the meantime, make the filling. Combine the rhubarb, strawberries, sugar and orange juice. Set aside.
Remove the dough from the fridge and turn out onto a floured surface.
Divide it in half and roll into two discs large enough to fit into the pie dish with a little over hang.
Press one disc onto the base of pie dish and spoon in the strawberry and rhubarb filling.
Cut the other pastry disc into strips and place over the filling in a lattice pattern.
Lightly brush the pie crust with the egg and place in the oven for 35 minutes until the filling is soft and the crust is crispy.
Leave to cool before serving.