Preheat your oven to 400°F. Roll one pie crust on a floured surface into a 12-inch circle. Place it into a 9-inch pie plate, gently pressing it into the bottom and sides.

In a mixing bowl, combine sugar and cornstarch. Add sliced strawberries and rhubarb, stirring until they're well coated.

Pour the fruit mixture into the pie crust, spreading it evenly. Roll the second pie crust into a 10-11 inch circle and set it aside.

Beat the egg with water in a small bowl. Brush the edges of the bottom crust with the egg wash.

Place the second crust over the filling, aligning it properly. Trim any excess dough, leaving about ½ inch overhang. Tuck the overhang under the edge of the bottom crust and crimp the edges together.

Cut slits in the top crust for steam to escape. Brush the top crust with the remaining egg wash.

Bake the pie in the preheated oven at 400°F for 20 minutes. Reduce the heat to 350°F and bake for another 40-50 minutes until the filling is bubbly and the crust is golden brown.