Preheat: Preheat the oven to 400 degrees.
Prep Ingredients: Zest the lemon. Prepare the rhubarb and strawberries. See note 1.
Make Filling: Add the rhubarb, strawberries, sugar, cornstarch, lemon zest, lemon juice, vanilla extract and salt to a large bowl and stir until well combined. Set aside.
Bottom Crust: Unroll one of the pie crusts and put it in the bottom of the pie dish. Press the crust against the bottom and sides of the pie dish. It is ok to have extra pie crust hanging over the edge. In fact, it will make Step 6 easier.
Add Filling: Pour the filling into the crust-lined pie dish. Cut the butter into small pieces and distribute evenly over top of the filling (see 0:27 of video for reference).
Top Crust: Gently place the second pie crust on top of the pie.*** Then pinch the edges of the bottom crust and top crust together to seal (see :40 of video). Fold over or trim excess crust. Use your thumb to press a decorative boarder around the edge of the pie (also known as fluting). Cut slits in the top of the pie crust to allow for venting during baking. Beat the egg in a small bowl and brush onto pie crust. Sprinkle the pie with a little sugar (1/4-1/2 Tbsp.).
Bake: Using 2-3 inch wide strips of foil, cover the edge of the pie to prevent it from burning. Put the pie on a baking sheet and place in the oven for 35 minutes. Remove the foil and return the pie to the oven for an additional 10-15 minutes for a total time of 45-50 minutes.