Preheat oven to 375° F. In a large bowl, toss together apples, cranberries, sugar, cornstarch, lemon zest, cinnamon, and salt.
Transfer the mixture to the piecrust. Top with granola and dot with butter.
Bake until the apples are tender, 50 to 60 minutes. (If the top darkens too quickly, tent loosely with foil.) Let cool before serving.