Mix together the rhubarb, strawberries, mint, white and brown sugar, lemon, salt and tapioca in a large bowl. Let sit until tapioca is starting to soften and juices are extruding from the fruits, at least fifteen minutes.
Preheat oven to 400 degrees
Roll out half your pie crust and line a deep-dish pie tin with it.
Roll out the other half and cut into long strips about 1” wide.
Fill the pie tin with the fruit and dot with the butter.
Using the strips of pie crust, weave a lattice on top of the pie by placing half of the strips at 1-2” intervals across the pie. Fold every other strip back halfway (to the center). Then lay a strip across the pie in the center covering the strips which have not been folded back. Return the folded strips to straight (going over the strip under them) and fold back the other strips. Lay a piece over the pie again and repeat. Continue, repeating on the other side, until finished. Fold up the bottom edge to seal in the lattice weaves.
Brush the entire pie crust with egg wash and sprinkle the pie with Demera sugar.
Bake at 400 degrees for 20 minutes then reduce oven temperature to 350 degrees and cook for 20-30 minutes until pie is golden brown and juices are bubbling up.