Prepare the Filling: Preheat your oven to 375°F (190°C). In a large bowl, combine the rhubarb, strawberries, granulated sugar, cornstarch, lemon zest, lemon juice, vanilla extract, and salt. Stir until the mixture starts to become juicy.
Form the Base: Roll out one batch of the pie dough on a lightly floured surface to a 12-inch diameter round. Fit this into a 9-inch glass pie dish.
Add the Filling: Pour the fruit mixture into the pie crust.
Create the Lattice: Roll out the remaining dough into a 12-inch round and cut into 10 strips, each about 1 inch wide. Weave these strips over the filling to form a lattice. Fold the edges of the bottom dough over the lattice edges and press to seal.
Finish and Bake: Dot the pie with pieces of butter. Brush the lattice with the beaten egg and sprinkle with sanding sugar. Bake the pie, covering with foil during the last 30 minutes to prevent over-browning. Bake until the crust is golden and the filling bubbles, about 70 to 80 minutes.
Serve: Let the pie cool slightly before serving with vanilla ice cream.
