In a medium saucepan, whisk together the sugar, cornstarch, and salt, then add the reserved cherry juice from the canned cherries.
1 cup (200g) granulated sugar, 1/4 cup cornstarch, 1/8 teaspoon salt
Bring the mixture to a simmer over medium heat, stirring frequently until thick and bubbly and the cloudy liquid turns into a clear gel, about 5 to 7 minutes.
Stir in the cherries, butter, and food coloring, if using, then cool completely before adding to the pie crust. This can be done up to 1 week in advance and stored in the fridge in an airtight container.
44 ounces tart cherries, 2 Tablespoons salted butter, Red food coloring
Preheat oven to 425°F.
On a lightly floured surface, roll out the bottom pie crust and use it to line a 9-inch deep dish pie plate. Spoon the cherry pie filling into the bottom crust.
1 [double pie crust recipe]
Roll out the remaining crust and use it to top the cherry pie filling. You can make a lattice crust, or just do a solid crust with slits cut in the top using a sharp knife for venting. Be sure to seal the edges, either by crimping or fluting, then brush the top crust with the beaten egg and sprinkle with sugar, if desired.
1 beaten egg, Additional sugar
Bake for 10 minutes at 425°F, then decrease the oven temperature to 375°F and finish baking for 45-50 minutes, until the crust is golden brown on top and the filling is bubbling. Check the pie around 25 minutes and cover the edges with a pie protector or tinfoil if they are starting to brown too much.
Remove the pie from the oven and cool for at least 3-4 hours before serving.