Melt 1 tbsp of butter, add the sliced apples and fry them on medium heat for 3-4 minutes.
Add the blueberries and sugar, fry for 2 minutes and put aside to cool down. If the apples you use are sour, you may want to add more sugar.
Grease an ovenproof dish (I used a round one, 23 cm diameter) and transfer the prepared fruit to the dish.
Put the flour and the baking powder to a bowl, add the butter pieces and with your fingers combine them together till the mixture resembles fine crumbs.
Stir in the sugar and lemon zest. If you are preparing this dessert in advance, you would continue with the next steps approx. 45 min. before you would like to serve it.
Preheat an oven to 200 Celsius degrees (180 degrees for fan ovens).
Add the milk to the dry mixture and stir with a spoon until everything is combined.
Using two teaspoons put the mascarpone on top of the fruit, and then the same way spoon the prepared batter on top of the mascarpone (you don't have to smooth the layers as mascarpone will melt in the oven, and the batter will rise).
Bake in the oven for 20-25 minutes.
Let it rest for 10 minutes before serving.