Preheat the oven to 350F.
In a medium-sized bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large bowl, whisk together the oil, sugar, egg, vanilla, and sourdough discard. Set aside.
Prep your cake pan by spraying the pan with oil spray. Then cut out a circle of parchment paper that fits the bottom inside of the pan snugly. Place the circle in the pan and spray the top of the paper with oil.
Peel the apple(s) and slice finely.
Evenly press the brown sugar onto the paper at the bottom of the pan so it covers the bottom entirely to the edges. Lay out the apple slices by overlapping them until they make a row around the outside.
Then continue with rows to the center until you fill the whole surface area. Any leftover apple slices can be placed on top somewhat haphazardly because they won't show after baking. (I usually end up making two layers, and the second one is with no overlapping.)
After your pan is prepared with the brown sugar and apples, it's time to mix the batter. Dump the flour mixture into the wet mixture bowl and fold just until no lumps appear. Then scoop the batter on top of the apples in the pan and even it out carefully, as not to disturb the apples beneath it.
Bake the cake for 1 hour to 1 hour and 10 minutes, or until a toothpick comes out clean in the center. Let cool in the pan for 10-15 minutes, then place your serving plate upside down on top of the pan and flip the cake onto the plate. You should hear it plop down. Remove the pan, and remove the paper atop the cake. Wait 15 minutes to serve and slice if you want to eat it warm! Enjoy!