Begin by preparing your pie crust. Roll out both the top and bottom. Place the bottom crust in the pie dish and set the top aside. (I like a lattice crust for this pie. You can learn how to create a lattice effect here.)
Next, preheat your over to 350 degrees F.
In a large bowl, mix together the rhubarb, strawberries, sugar, flour, one egg, milk, and vanilla. Mix well and pour into the bottom crust. Sprinkle cubes of butter on top of the filling.
Place the top crust over the filling or cut the crust into strips to create a lattice effect. Crimp the edges to seal. Then brush the top crust with your egg wash and sprinkle with coarse sugar.
Bake for 1 hour or until the crust is golden and the filling is bubbling.
Let cool before serving with a big scoop of vanilla ice cream.