How to Make It
Preheat oven to 350°F. Line a 9 by 13-inch baking dish with two crisscrossed pieces of parchment paper, leaving an overhang on all sides. Coat the parchment with nonstick cooking spray.
Whisk together 3¼ cups flour, ½ cup granulated sugar, and 1 teaspoon salt in a large bowl. Slowly add melted butter and stir with a fork and press gently with your hands until combined. Remove 1 cup of mixture; set aside. Press remaining mixture into bottom of prepared pan. Prick all over with a fork. Bake until puffed and lightly browned, 18 to 20 minutes, rotating halfway through.
Meanwhile, combine Granny Smith apples, Golden Delicious apples, cranberries, cornstarch, cinnamon, nutmeg, remaining 1¾ cups granulated sugar, 2 tablespoons flour, and ¼ teaspoon salt in a large bowl. Transfer to cooled crust.
Stir oats into 1 cup reserved crust mixture. Pinch mixture into small pieces and scatter topping mixture over the filling. Top with turbinado sugar, if using. Bake until topping is golden brown and the filling is bubbling, 60 to 70 minutes, rotating halfway through. Let cool at least 2 hours before slicing. Serve with ice cream or Greek yogurt, if desired.