In a medium saucepan over medium heat, add cherries. Cover and cook, stirring frequently, until the natural juices of the cherries are released and the contents are beginning to simmer (about 10-15 minutes).
In a small mixing bowl, whisk together cornstarch and water.
Reduce heat to low and pour the cornstarch slurry, sugar, and lemon juice into the cherries. Stir to combine.
Bring the mixture back to simmer and continue cooking until the mixture has thickened. (This happens fast, 1-2 minutes.)
Remove from heat and allow to come to room temperature before using. You will use the entire amount (about 3 cups) in the pie.
Pie Assembly
Preheat the oven to 400°F.
When ready, roll out one of the pie crusts into a 12-inch circle. Place into a 9-inch pie pan.
Top the pie crust in the pan with the cooled cherry pie filling.
Set out an 18-inch piece of parchment paper and roll out the second pie crust to a 14-inch circle, about ⅛-inch thick. Using a pizza cutter, slice into 2 to 2½-inch strips.
Cover with a latticework crust by arranging half the strips over the pie, in parallel lines, leaving space in between.
Fold every other strip up to about the mid-line of the pie. Lay another strip over the middle of the pie, perpendicular to the others. Fold the parallel strips back down, then fold up all the other strips. Lay another perpendicular strip down, and repeat until all the strips have been used.
Trim any excess dough and press to adhere to the bottom crust. Using a fork, crimp the edge of the pie to seal.
In a small bowl, mix together egg and water. Brush the latticework crust with the egg wash mixture and then sprinkle with turbinado sugar.
Bake 40-45 minutes or until cherry filling is starting to bubble.
Cool completely before serving.
Notes
You can absolutely use store-bought cherry pie filling.