In a large bowl, toss together the cherries (if using frozen cherries, they should be frozen; do not thaw), sugar, cornstarch, and salt.
Let the cherry mixture sit for about 20 minutes if using fresh cherries, 60 minutes if using frozen cherries. (If your cherries have developed a lot of juice, you will need to cook the mixture in a saucepan on simmer until the sauce thickens, about 5 minutes; let cool.)
Remove your dough from the fridge. Roll one piece into a circle large enough to fill a 9-10" pie place and fit gently into plate. DO NOT stretch your dough!
Dump the cooled cherry filling into the crust-lined pie plate. Dot the top with the butter evenly over the filling and then make your lattice crust (See notes; easier than you think!)
Trim the dough, roll the edges under and crimp the edges with your fingers or a fork then lightly brush the entire surface of the pastry with the beaten egg. Sprinkle with some sugar; I love using large crystal sugar typically used for cake decorating.
Place the pie on a rimmed baking sheet lined with aluminum foil.
Bake the pie at 425°F (220°C) for 15 minutes. Turn the oven down to 375°F (190°C) and bake until bubbling, at least another 30 minutes (longer if using frozen cherries).
Let the pie cool slightly before slicing and serving.