Preheat the oven to 425°F
Sprinkle the bottom of a 9-inch pie pan with flour
Fit one pie crust into the pan
Trim the edges about 1/2 inch beyond the rim
Fold the overhanging crust under itself to create a thick edge
Refrigerate while preparing the filling
In a large bowl stir together the rhubarb, and strawberries
Mix the sugar and cornstarch together, and then stir them into the rhubarb and strawberries
Remove the chilled pie crust from the refrigerator
Spoon the filling into the pie crust, if there is any extra sugar cornstarch mix left just sprinkle it over the top of the rhubarb
Dot the filling with the butter
Place the second pie crust over the filling
Trim the top crust about 1/4 inch beyond the edge of the pie pan
Brush a little water around the edge to seal the top and bottom crusts together
Crimp the edges as desired, I use my thumb and forefinger to pinch the crusts together
Cut several steam vents in the top crust
If the pie dough has become warm, chill the pie for 10 minutes
Whisk the egg with a splash of water and brush over the top crust
OPTIONAL Sprinkle with sugar, if using
Place the prepared pie on a lined baking sheet to catch any juicy dribbles
Bake at 425°F for 15 minutes
Reduce the oven temperature to 375°F and bake for another 40-50 minutes, or until the crust is deeply golden and you can see the filling is bubbling through the vents
Cool completely on a wire rack before slicing for the cleanest slices
Serve at room temperature or slightly warm