Preheat oven to 400 degrees.
Add cold, unbaked pie shell to a 9" pie pan.
Stir the rhubarb, strawberries, sugars, lemon, salt and tapioca together in a large bowl.
Mound filling inside bottom pie crust and dot the top with small pieces of butter.
Cut the remaining pie crust into 3/4" wide strips. Add the strips in a lattice pattern over the filling. If using a round premade crust, start in the center with the longest pieces, ending with the smaller pieces. Extend the pieces slightly over the edge. If you have any crust left, cut into shapes and add to top of lattice. This is optional.
Fold the lattice stips under the bottom pie shell and crimp the edge. Brush heavy cream on the top crust. Sprinkle lightly with sanding sugar or coarse sugar (optional).
Transfer the pie pan onto a baking sheet to catch any juice that bubbles over the edge.
Bake for 20 minutes. Reduce the oven temperature to 350 degrees and bake for an additional 25-30 minutes, until the crust is golden and the juices bubbly.
Remove from oven and cool on a wire rack until room temperature. this will take 2-3 hours. The juices will gel while the pie cools. If desired, lightly sprinkle confectioner's sugar over the crust cut-outs. Serve at room temperature or chilled.