Prep the Filling. In a large bowl, mix together the diced rhubarb, sliced strawberries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Stir until the fruit is evenly coated.
Let Sit. Allow the mixture to sit for about 15 minutes to let the flavors meld and the cornstarch to begin absorbing the juices.
Assemble the Pie. Roll out one pie crust and fit it into a 9-inch pie dish. Trim the edges to leave about a 1-inch overhang. Pour the fruit filling into the prepared pie crust, spreading it evenly. Dot the filling with small pieces of the diced butter. Roll out the second pie crust and place it over the filling. Trim the edges, leaving about a 1-inch overhang, and crimp the edges to seal.
Add Egg Wash. In a small bowl, whisk together the egg and water. Brush the top crust with the egg wash to create a golden, glossy finish.
Bake the Pie. Preheat your oven to 375°F. Place the pie on a baking sheet to catch any drips and bake for 50-60 minutes, or until the crust is golden brown and the filling is bubbling.
Cool. Allow the pie to cool completely on a wire rack before slicing.