2) Whisk the egg, vinegar and water in a measuring jug and pour over the dry ingredients incorporating all the liquid without overworking the dough. Form the dough into a ball, toss the additional plain flour over it and chill.
3) Divide the dough into 2 disks. Roll out 1 piece to make a bottom crust and place into a 20 cm round pie dish. Put dish in refrigerator to chill.
4) Preheat oven to 210C/Gas 7
5) Mix the rhubarb, strawberries, sugar, semolina, plain flour, zest and juice of lemon, dash of cinnamon, and vanilla together in a large bowl and pour out into the chilled crust. Dot the top of the filling with the butter.
6) Brush edges of pastry with egg white wash. Roll out the other piece of dough and place over filling and crimp to seal edges.
7) Brush with egg white wash and sprinkle with granulated sugar. Collar with foil, place on centre shelf of oven and bake for 15 minutes.
8) Decrease temperature to 190C/Gas 5 and bake for an additional 45 to 50 minutes, or until the filling starts bubbling. Cool before serving.