In a medium bowl, mix together the rhubarb, pear, brown sugar, corn starch, lemon zest, vanilla extract and salt. Allow to stand, about 10 to 15 minutes, while you assemble the pie crust.
Pour the filling into the slab 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 few wide strips of pie dough, weaved with a few strands of braided pie dough. Get as fancy as simple as you like. Tuck the edges under and crimp around the entire slab pie. Transfer to the freezer to chill for 30 minutes. Meanwhile, preheat the oven to 400 degrees F. Brush the top of the pie with a generous amoutn of egg wash and sprinkle with about a teaspoon of turbinado sugar.
Place the slab pie on a half 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? Nah. Serve with ice cream or whipped cream.