Preheat oven to 350 F. Grease a 9x13-inch baking pan with 2 tablespoons of butter.
For the apple pie filling, combine the apples, brown sugar, cinnamon, nutmeg, cornstarch, and lemon juice in a large bowl. Spread it into the prepared pan in an even layer.
For the biscuit topping, whisk the flour, sugar, baking powder, baking soda, and salt together in a large bowl. Add the cold butter pieces, and using your fingers or a pastry cutter, cut the butter into the mixture until it resembles coarse crumbs. Stir in the buttermilk until combined, but do not over mix — the dough will be lumpy.
With a large spoon, dollop 10 spoonfuls of the biscuit batter over the apples. The apples will show in between the batter, so do not spread. Sprinkle the cinnamon sugar topping evenly over the biscuit batter, optionally.
Bake until the apples are bubbling, and the top is golden brown, approximately 40 to 45 minutes. Place on a wire rack, and let cool for 10 minutes before serving.