Preheat the oven to 350°F (180°C) and line a 5 inches (12 cm) round baking mold with parchment paper or grease it with oil.
Wash, core, and optionally peel the apple. Cut it into chunks.
In a high speed blender, combined the diced apple with egg, Greek yogurt, rolled oats, maple syrup, ground cinnamon, vanilla extract, baking powder and a pinch of salt.
After blending, you can stir in additional ingredients like apple chunks or chopped walnuts.
Let the batter rest in the fridge for 10 minutes.
Transfer the mixture to your prepared baking mold, spreading it out evenly. If desired top with thin slices of apple.
Bake in the middle rack of the hot oven for 25-30 minutes, or until the cake has set and is cooked inside. To check for doneness, insert a toothpick into the center: it should come out clean with just a few crumbs.
Let cool for at least 10 minutes before slicing and serving.