Butter a 2-quart casserole; set it aside. In a medium saucepan, combine pie filling, peaches, and apples. Bring to just boiling. Stir in cinnamon. Pour into prepared casserole. Dot with 2 tablespoons butter. Cover; keep warm.
In a mixing bowl, combine flour, sugar, cornmeal, baking powder, and salt. With a pastry blender or 2 knives, cut in remaining 2 tablespoons butter until pieces are the size of small peas.
Stir in buttermilk, lemon peel, and lemon juice. Drop by rounded tablespoonfuls onto hot fruit mixture. Sprinkle with additional sugar.
Bake, uncovered, in a 400 degree F oven for 20 to 25 minutes or until golden brown. Serve with whipped cream or ice cream and garnish with mint sprigs, if you like.