1. Pit and halve fresh cherries if using them. Place cherries in a saucepan with sugar and lemon juice over medium heat until cherries release their juices. Using a slotted spoon, transfer cherries to a bowl.
2. Stir cornstarch into some of the cooking liquid until smooth, return it to the pot, and cook gently until thickened. Add butter and stir until melted. Pour the thickened sauce over the cherries and fold gently to combine. For canned cherries, drain and reserve juice; mix cornstarch with some of the reserved juice, cook with sugar to thicken, then fold in cherries, butter, and lemon juice. Optionally add red food coloring for richer color. Cool filling to room temperature.
3. Preheat oven to 400°F (205°C). Roll out one pie crust and fit it into a 9-inch pie dish. Pour in cherry filling and sprinkle cinnamon over the top. If using fresh cherries, dot the surface lightly with cold butter cubes.
4. Roll out second pie crust to create either a full top crust with a vent or cut into strips to create a lattice pattern. Seal and crimp edges securely.
5. Brush the top crust with beaten egg white and sprinkle granulated sugar for sparkle and crunch if desired. Chill the assembled pie briefly (20-30 minutes) to help set.
6. Bake the pie on a baking sheet at 400°F (205°C) for 20-25 minutes. To prevent over-browning, cover the crust edges with foil after 25 minutes. Reduce heat to 375°F (190°C) and bake another 20-25 minutes, until the crust is golden and filling bubbles.
7. Cool the pie on a wire rack for several hours or refrigerate overnight to allow filling to set properly before slicing.