Preheat the oven to 175 degrees Celsius.
Prepare 6-7 muffin tins.
Add egg, apple sauce, Greek yogurt and granulated sweetener to a big bowl and whish until slightly fluffy and combined.
Combine oat flour, coconut flour, cinnamon, baking powder, baking soda and salt in a separate bowl.
Add the dry ingredients little by little and whisk together.
Core and dice the apples, fold into the muffin batter.
Add cream cheese and Greek yogurt to a bowl and whisk until fluffy and smooth.
Add sweetener and egg yolk and whisk just until combined.
Combine ingredients for the crumble if using.
Fill the muffin tins half ways, add about 1 tbsp of filling in each tin, make a swirl using a knife, then top with the rest of the batter.
Add the crumble on top and press down gently (if using).
Bake for 16-20 minutes, or until an inserted toothpick comes out clean.
Let them cool down for 20-30 minutes.