In a medium bowl, mix together the rhubarb, apple, brown sugar, vanilla bean, corn starch and salt. Allow to stand, about 10 to 15 minutes, while you assemble the pie crust.
Pour the filling into the prepared pie crust (see above for assembling instructions). Roll the second half of dough and make any sort of design that you like. I chose a very tightly weaved lattice. For this, I cut the dough into 1/2-inch strips using a ravioli cutter. I weaved a lattice, tucked the pie edge under itself all the way around and then crimped the dough all the way around.
Brush the tops of the pie with a generous amount of egg wash and sprinkle with about a teaspoon of turbinado sugar. Transfer to the freezer to chill for 15 to 20 minutes. Meanwhile, preheat the oven to 400 degrees F.
Place the pie on a baking sheet and transfer to the oven to bake for about 20 minutes, until the crust is lightly golden brown. Turn the oven's heat down to 350 degrees F and bake for an additional 20 to 30 minutes, until the crust is golden brown and the filling is bubbling. Transfer to a cooling rack and allow to cool to room temperature, about 3 hours. If you cut into it before cooling it down, the filling will spill out, but is that the worst thing in the world? No. Serve with ice cream or whipped cream