Pie Dough (If using a pre made or other pie dough recipe, skip to next section)
Dice your butter and place on a plate in the freezer.
Combine flour, salt, and sugar, in a large bowl. Remove butter from the freezer and add into flour. Work with your fingers or a pastry cutter until it resembles coarse crumbs.
Add in the teaspoon of apple cider vinegar and two tablespoons of water. Begin mixing, adding 1 tablespoon more of water at a time until the dough begins to come together but is still loose.
Separate into two, form each half into a flat disc and wrap in plastic. Chill for one hour.
Assembling your pie
While dough is chilling, pit and slice your cherries in half and dice your rhubarb into 1/4 inch pieces. Combine in a bowl and set aside.
On a floured surface, roll out one disc of pie dough until it's several inches wider than your pie plate. Let rest a few minutes, then lift gently into your pie plate.
Lifting from the edge, instead of pulling, press the pie dough into the pie plate. Place the pie plate into the fridge.
Preheat your oven to 350°F. Toss the cherries and rhubarb with the sugar and cornstarch. Set aside.
Roll out your second pie dough and cut shapes or slits in the middle of it.
Take out bottom pie crust and pour in filling. Dice the remaining butter over the top of the filling and place the top pie crust over the top.
Firmly crimp the edges then trim off any excess pie dough, leaving 1/8 inch of overhang from the rim of the pie plate.
Place the whole pie in the freezer for 10 minutes*. Meanwhile, beat the egg with 1 teaspoon of water.
When pie is chilled, brush with egg wash and place immediately in hot oven for 1 hour and 10 minutes, or until golden brown and the filling is bubbly.
Let cool completely before slicing. Serve slightly warm or room temperature with whipped cream or ice cream. Store remaining pie covered at room temperature for up to 3 days.
Notes
*If using a different pie crust that uses shortening, there's no need to chill your pie in the freezer before baking.*If crust begins to brown too much on the edges during baking, use a pie ring or foil to cover it for the remaining bake time.