Begin by preparing the cherry filling: in a large mixing bowl, gently toss the pitted cherries with sugar, cornstarch, and lemon juice until evenly coated. Set aside to let the flavors meld.
Roll out one pie crust on a lightly floured surface to fit your pie dish, then transfer it carefully into the dish, pressing gently to fit the bottom and sides.
Pour the prepared cherry filling into the crust, spreading it out evenly with a spoon or spatula. Dot the filling with small pieces of butter to add richness.
Cover the filling with the second pie crust, trimming any excess dough and crimping the edges together to seal. Cut a few small slits in the top crust for steam escape.
Brush the top crust with the beaten egg to give it a shiny, golden finish. Place the assembled pie on a baking sheet to catch any drips, then put it in a preheated oven at 375°F (190°C).
Bake for about 45 minutes, or until the crust is golden brown and the filling is bubbling thickly through the slits. Keep an eye on the edges to prevent burning; if they brown too quickly, shield with foil or a crust shield in the last 15 minutes.
Once baked, remove the pie from the oven and let it cool for at least 2 hours to allow the filling to set. Serve slices warm or at room temperature and enjoy the luscious, jammy cherry goodness with a flaky crust.
