Heat oven to 400° F. Roll the piecrust into a 13-inch circle and fit it in a 9-inch pie plate, allowing the extra pastry to drape over the edge.
In a medium bowl, combine the cherries, comstarch, sugar, and cinnamon; spoon into the pastry and scatter with the pistachios. Fold the extra pastry over the filling, pleating it so that it lies flat. Brush the pastry with the egg and sprinkle with additional sugar (about 1 tablespoon). Bake 40 to 45 minutes or until the crust is golden. Cool on a rack for 45 minutes. Serve with the ice cream.
Note: If using Bing cherries, reduce the sugar to 3/4 cup.