Preheat oven to 350°F; grease a 9x13-inch dish.
Toss apples with 1/2 cup sugar, 1 teaspoon cinnamon, nutmeg, and 2 tablespoons flour in a large bowl. Spread evenly in dish.
Whisk 1 1/2 cups flour, 1/2 cup sugar, brown sugar, baking powder, cream of tartar, and salt in another bowl.
Stir melted butter and buttermilk into dry ingredients just until combined. Drop large batter spoonfuls over apples; spread slightly, leaving gaps.
Sprinkle generously with cinnamon-sugar.
Bake 42-47 minutes until topping is golden brown and apples are knife-tender. Cool 10 minutes before serving.