Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

Let the pie cool for at least 2 hours before cutting and serving.