Combine sugar and flour in a big bowl.
Add the chopped rhubarb and strawberries. Mix with flour and sugar, then set aside for half an hour.
Fill pie shell with prepared filling. Spread butter on top, then place the top crust on top. Use water to seal the top and bottom crust's edges.
Apply yolk with pastry brush to pie top.
Sprinkle with sugar.
Make tiny incisions on top to allow steam to escape.
Bake for 35 to 40 minutes, or until bubbly and brown, at 400 degrees Fahrenheit (200 degrees Celsius). On a rack, cool.