Preheat oven to 400.
Hull and chop the strawberries and place in a large bowl.
Chop the rhubarb into small pieces and add to the strawberries.
Add the sugar, cornstarch and vanilla to the strawberry mixture and mix to combine. Set aside.
Press one pie crust into a pie dish.
Mix strawberry mixture again and spoon into pie dish. Leave excess liquid at bottom of bowl out of pie dish.
Smooth out filling to level.
Evenly spread butter pats over top of filling.
Slice remaining pie crust into strips and weave a lattice on top of filling.
In a small bowl, whisk together egg yolk and water.
Brush lattice with egg wash.
Bake at 400 for 20 minutes.
Reduce heat to 375 and bake 20 minutes.
Tent the top of pie with aluminum foil and bake another 20-30 minutes.
Allow to cool before slicing.