In a bowl mix well: yoghurt, whole eggs, lemon zest and sugar ( you can use an electric mixer).
Once the ingredients are well mixed, start adding the flour and baking powder and mix very well in order to get a homogenous dough.
Peel the apples and slice them into 1-2 mm slices (when too thin they will loose their texture during baking).
Add the apples to the dough and stir well.
Lay a round baking tin ( 23cm) with baking parchment. Transfer the dough into the tin, sprinkle with the brown sugar and bake for 60 min in an oven heated to 180C ( if fan assisted oven lower the temperature to 170C)
After 40 minutes of baking you can cover the cake with a layer of an aluminium foil in case the cake browns too quickly.