In a large bowl, combine the rhubarb, strawberries and sugar together and let sit for 30 minutes.
Heat the oven to 425° and spray a pie pan with cooking spray.
Unroll one of the pie crusts and lay in the pie pan, pressing into the edges.
Stir the cornstarch, salt and vanilla into the rhubarb, then pour into the pie crust. Place small pieces of the butter around the the top of the filling.
Unroll the second pie crust and cut strips, then weave them into a lattice pattern across the top of the filling (or leave it uncut and just cut a few slits in it for steam vents).
Use 2 fingers on the outside of the crust and press your thumb between them to seal and crimp the edges together.
Beat the egg and milk together and brush over the pie crust.
Place pie in the oven on the baking sheet for 15 minutes.
Remove the pie and cover the edges of the crust with foil. Reduce the oven temp. to 350°F and place the pie back in the oven to bake for an additional 40 minutes, or until the crust is golden brown and the filling is bubbling.
Remove from the oven and cool for 3 hours before serving.
Notes
Storage
Cover well with plastic wrap and store in the refrigerator for 7 days or on the counter for 3 days. Freeze up to 3 months.