Heat the oven to 375°F and butter a 9-by-13-inch baking dish.
Toss the apples and frozen cranberries with 1 cup sugar, cornstarch, orange zest and 1/4 teaspoon salt.
Spread the fruit in the dish and bake for 12 minutes.
Whisk the flour, 1/3 cup sugar, baking powder and 1/2 teaspoon salt; cut in the butter and stir in the buttermilk.
Drop the dough over the hot filling in nine rough mounds.
Bake for 34 to 38 minutes until the biscuits are golden, the apples are tender and the center bubbles; cool 30 minutes.