Preheat the oven to 180oC/Fan 160oC/Gas Mark 4. Grease the base and sides of a 900g/2lb loaf tin, then line with non-stick baking parchment.
Sift the flour, baking powder and caster sugar into a bowl. Beat the Quark, egg yolks, lemon zest and oil together in a jug. Use an electric whisk to whisk the egg whites together in a clean bowl until they form soft peaks.
Stir the Quark mixture into the dry ingredients, then stir in a third of the egg whites. Next, add the remaining egg whites and gently fold into the mixture using a figure of eight motion.
Spoon the mixture into the prepared tin and then bake in the middle of the oven for 50-55 mins or until risen and firm to the touch. When lightly touched in the centre the cake should bounce back. Leave the cake to cool in the tin for 10 mins.
Mix the juice of the lemon with the granulated sugar and spoon over the top of the cake, whilst it is still warm. Leave the cake to cool in the tin standing on a wire rack. Once cold, remove from the tin, peel away the lining paper and serve in thick slices. This cake will keep well in an airtight container for up to a week.