Whisk cornstarch and 1 tablespoon water together in a large bowl until combined; stir in rhubarb, strawberries, white sugar, vanilla extract, cinnamon, and lemon juice. Set filling aside for 30 minutes.

Preheat the oven to 425 degrees F (220 degrees C).

Roll out half dough on a floured surface to fit a 9-inch pie plate; place in pie plate and refrigerate until ready to use. Roll top crust on a floured surface into a 10-inch circle. Cut into 1-inch wide strips with a sharp paring knife or pastry wheel; set aside.

Stir filling; pour into the prepared pie plate. Moisten the rim of the crust with a small amount of water. Start with the longest dough strips and lay the first 2 in an X in the center of the pie. Alternate horizontal and vertical strips, weaving them in an over-and-under pattern. Use the shortest strips for the edges of the lattice. Press strips down on crust rim to seal; trim excess. Beat egg white and 1 teaspoon water together in a small bowl; brush over lattice top. Sprinkle with turbinado sugar. Wrap aluminum foil strips around edges of pie.

Bake in the preheated oven for 15 minutes. Reduce the oven temperature to 375 degrees F (190 degrees C); bake until crust is golden and filling is bubbling, 40 to 45 more minutes. Remove aluminum foil for the last 10 minutes of baking time. Cool pie completely before serving.