Preheat oven to 375°F (190°C).
In a large bowl, combine strawberries, rhubarb, sugar, cornstarch, salt, vanilla extract, and lemon juice. Mix well.
Roll out one pie crust and place it in a 9-inch pie dish.
Pour the fruit mixture into the pie crust.
Roll out the second pie crust and place it over the fruit mixture. Trim excess dough and crimp edges to seal.
In a small bowl, beat the egg with water to make an egg wash. Brush the top crust with the egg wash.
Cut a few slits in the top crust to allow steam to escape.
Bake for 1 hour, or until the crust is golden brown and the filling is bubbly.
Let the pie cool before serving.