Preheat the oven to 175 degrees Celsius.
Line or grease a 18 or 20 cm spring form (I used a 18 cm).
Combine oat flour, rice flour, cinnamon, baking powder, baking soda, salt in a bowl. Set aside.
Add eggs and granulated sweetener to a big bowl. Whisk until fluffy and slightly pale (takes a few minutes).
Add the mashed banana, apple sauce and greek yogurt to the bowl and whisk until incorporated and smooth.
Add the dry ingredients in three steps, gently whisking in between to incorporate the dry ingredients into the batter.
Dice 1 1/2-2 apples (depending on how large they are), you should have about 2 dl diced apples. Add to the cake batter and gently fold in.
Pour the cake batter into the prepared spring form.
Slice 1 1/2-2 apples thinly and arrange on top of the batter.
Place in the middle of the oven, bake for 15 minutes.
Meanwhile prepare the caramel sauce if making your own by adding granulated sugar and water to a small pot. Bring to a boil, lower to medium heat and let it simmer until the sugar has caramelized (takes around 4-5 minutes). Remove from the heat, let it cool for one minute. Stir in the cream whisking constantly until incorporated.
Pour the caramel sauce over the cake, spreading it gently over the cake, then bake for another 25-30 minutes, until golden brown or when a toothpick comes out clean.
Remove from the oven, let it cool for 10 minutes. Run a knife around the edges then gently remove from the spring form. Place on a cooling rack at let it cool for at least 20-30 minutes more. Serve slightly warm or cold.