Prepare the pie crust if using homemade but don't bake. Chill while preparing the peach mixture.
Preheat the oven to 425 degrees F.
Peel the peaches and cut into 1/2-inch chunks.
Get out a large mixing bowl and combine the peaches, sugar, cornstarch, lemon juice and ground cinnamon until combined.
Roll one pie crust on a floured surface until it is about 12 inches in diameter.
Place the crust inside a 9 inch pie plate. Allow the excess to hang over the edges.
Pour the peach filling inside the pie crust and smooth until it looks even.
Place pieces of butter all over the pie filling.
Roll the other pie crust until it is 12 inches in diameter. Cut into 1 inch strips. Use a knife or pizza cutter to easily do this.
Arrange the strips in a lattice pattern over the pie filling. Trim excess dough off the lattice strips.
Fold the bottom pie edge over the lattice edges so the pie looks neat. Crimp the edges with a fork.
Beat the egg in a small bowl. Use a pastry brush to brush the egg wash on top of the lattice. Sprinkle the top with coarse sugar if you are using it.
Cover the edges of the pie crust with a pie shield or you can use aluminum foil if you don't have one.
Bake the pie for 20 minutes. Then reduce the oven temperature to 375 degrees for 40-50 minutes.
Remove from the oven. Wait 4-5 hours before slicing to give the filling time to set.
Then the pie is ready to serve and enjoy!