Double crust pie dough (or your preferred pie crust)
2 red plums, halved and pitted
6 cups pitted, halved sweet cherries (or 6 cups pitted frozen cherries), divided
½ cup sugar
Pinch of salt
1 tbsp. freshly squeezed lemon juice
2 tbsp. instant tapioca, ground
Pinch of ground cinnamon
2 tbsp. unsalted butter, cut into ¼-inch pieces
1 large egg lightly beaten with 1 tsp. water
To make the filling, process the plums and 1 cup of the cherries in a food processor or blender until smooth, about 1 minute. Strain the the puree through a fine mesh strainer into a large bowl, pressing on the solids to extract as much liquid as possible; discard the solids. Add the remaining cherries, sugar, salt, lemon juice, tapioca and cinnamon to the bowl. Mix well to combine; let stand for 15 minutes.
Preheat the oven to 400˚ F and place a baking sheet on an oven rack. Line a 9-inch pie plate with a round of pie dough. Transfer the cherry mixture to the dough-lined pie plate. Scatter the butter pieces evenly over the fruit. Top with the second round of pie dough. Pinch the edges of the dough together in a fluted pattern. Brush the top evenly with the egg wash. Using a sharp paring knife, cut 8 evenly spaced slits in the top crust to allow steam to escape. Place the pie in the freezer for 20 minutes.
Place the pie on the preheated baking sheet and bake for 30 minutes. Lower the oven temperature to 350˚ F and continue baking about 30-40 minutes more, until the crust is golden brown and the juices are bubbling. Transfer to a wire rack and let cool to room temperature so the filling has time to set up and thicken, 2-3 hours. Slice and serve.