Prepare double-crust pie crust recipe. Roll out one half of the crust and cover the bottom of a pie dish, while reserving the other half of the dough for the top crust.
1 recipe double-crust pie crust
Mix the rhubarb, strawberries, sugar, tapioca, flour, lemon zest and juice, cinnamon, and vanilla together in a large bowl and pour out into prepared crust. Slice the butter into small chunks and place them around the top of the strawberry rhubarb filling.
2 1/2 cups rhubarb, 2 1/2 cups strawberries, 1 cup granulated sugar, 3 Tablespoons minute tapioca, 1 Tablespoon all-purpose flour, 1/2 teaspoon lemon zest, 1/2 teaspoon lemon juice, 1/2 teaspoon ground cinnamon, 1 teaspoon vanilla extract, 3 Tablespoons salted butter
Roll out the remaining half of the pie crust and gently place it over the filling either as a solid crust with slits cut into it for ventilation or by cutting into strips and laying them out in a lattice pattern. Crimp to seal edges.
Make an egg wash by beating the egg with 1 teaspoon of water in a small bowl, then brush it over the top of the pie with a pastry brush and sprinkle the top of the pie with a teaspoon or two of sugar.
1 beaten egg, 1 teaspoon water, Extra sugar for sprinkling on top of crust
Loosely cover the edges of the pie with foil and bake at 425 degrees F for 15 minutes. Decrease temperature to 375 degrees F and bake for an additional 45 to 50 minutes, or until the filling is bubbling and the crust is a golden brown.
Let the pie cool for at least 2 hours before cutting and serving.