Preheat oven to 200 degrees C. Place a pie dish onto a baking tray.
Mix the strawberries, rhubarb, sugar, tapioca and lemon juice in a bowl then let stand for 15 minutes, stirring occasionally. Fit a pastry case into a 23cm pie pan; pour the filling into the bottom pastry. Scatter pieces of butter over the filling, and place the top pastry onto the pie. Press the edges together and crimp with a fork to seal. Cut several slits into the top with a sharp knife.
Bake in the preheated oven until the pastry is golden brown and the filling is bubbling, 40 to 55 minutes.
If your supermarket does not have tapioca, you can find it at Indian shops or at Bin Inn.