Coat an 8-or 9-inch baking pan with non-stick cooking spray and place the apple pieces in the bottom. Add the lemon juice, brown sugar, cinnamon, nutmeg and two tablespoons of the cake mix. Toss to combine. Set aside.
Place the remaining cake mix in a medium bowl and add butter. Using a pastry blender, two forks or your fingers, cut the butter into the cake mix until the mixture resembles coarse sand. Scatter the cake mixture evenly on top of the apples and then pour the buttermilk over the cake mix – do not stir. Sprinkle the cake with cinnamon sugar and then cover the dish tightly with aluminum foil.
Bake the covered cake in a 350°F oven for 25 minutes. Remove the foil and continue baking for an additional 30 minutes or until the cake is puffed and lightly golden brown. Cool for at least 10 minutes before serving. Top each portion with whipped cream and a dusting of cinnamon if desired. This cake is best eaten the day it’s made, but it will keep in a covered container at room temperature for up to three days.