Heat the butter over medium heat until melted and slightly browned, add 2 tbsp agave or maple syrup and spread all over the pan.
Half the plums and layer at the bottom of the pan, place the blackberries between the plums to fill in the gaps.
In a bowl combine together, ground oats, quick cooking oats, baking powder, baking soda and salt.
In another bowl add the egg, milk, sweetener, applesauce, Greek yogurt and vanilla. Whisk it up well.
Pour the dry ingredients into the wet ingredients and gently stir to combine. Let the batter sit for 10 minutes.
Pour the batter all over the fruits and gently smooth the top.
Bake at 350F (180C) for 40 minutes.
Let it cool for at least 30 minutes and invert the cake onto a large plate.
Slice and serve with a spoonful of Greek yogurt.