Preheat oven to 400°F

Process plums and 1 cup of the cherries until smooth.

Strain 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 remaining cherries, sugar, salt, lemon juice, tapioca, and cinnamon to the bowl.

Mix well to combine; let stand 15 minutes.

Transfer cherry mixture to the dough lined pan.

Scatter butter pieces over the fruit.

Top with second round of pie dough.

Brush top evenly with egg wash.

Put pie in freezer for 20 minutes.

Bake for 30 minutes.

Lower oven temperature to 350°F and continue to bake for 30-40 minutes.

Let cool 2-3 hours.