Prepare the apple topping: slice 1/2 - 3/4 of the apple into very thin half-moon slices. Put the slices in a bowl and cover with plastic wrap, then microwave for one minute. Remove from microwave, remove plastic wrap, and let sit while you prep the rest. Cut little 1cm chunks with the remaining apple, to put directly into the cake batter.
In the meantime, preheat oven to 350 and grease a round 8" cake pan.
In a large bowl, whisk together the wet ingredients (applesauce, honey, egg, oil). Slowly add in the dry ingredients, and stir until everything is just combined. Lastly, fold in any remaining apple chunks, if using.
Pour the batter into your cake pan, and arrange the microwaved apple slices on top. Start with an outer circle, then an inner one.
Bake your cake for 30 minutes, and allow it to cool completely before removing from the pan. Serve immediately, or refrigerate and enjoy within 3 days!