Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a large bowl, whisk together the oat flour, vanilla protein powder, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt until well combined.
In a separate medium bowl, whisk the unsweetened applesauce, honey, eggs, Greek yogurt, vanilla extract, and melted coconut oil until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix.
Spread a thin layer of the batter evenly into the bottom of the prepared cake pan.
Arrange a layer of thinly sliced apples over the batter.
Pour half of the remaining batter over the apple layer and spread evenly.
Add another layer of apple slices over the batter.
Pour the remaining batter over the apples and spread to cover completely.
Top with the remaining apple slices arranged neatly.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the cake to cool in the pan for 15 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.