Grease a 21cm round spring form cake tin, line with baking parchment and flour the sides.
Beat the butter and sugar until pale and fluffy.
Mix in the eggs adding a little of the flour if the mixture curdles.
Sift in the remaining flour, baking powder and salt and stir in along with the lemon zest and juice and the grated apple. Be careful not to over-mix.
Pour into the cake tin and place in the oven. Bake for 40 to 50 minutes. You will know the cake is ready when a skewer inserted into the middle comes out clean. The cake will have a slightly crusty top.
Cool in the tin for 10 minutes then turn out carefully to finish cooling.
For the candied lemon slices
Put the sugar in a wide bottomed pan, a frying pan is best. Add 180ml water and heat over a medium heat, stirring occasionally, until the sugar is dissolved.
Meanwhile, cut the lemons into rounds 2-3mm thick and discard the seeds.
Once the sugar has dissolved, place the lemon slices into the syrup making sure they do not overlap.
Simmer gently for 15 to 30 minutes, turning occasionally, until the skin and pith are translucent.
Cool on baking parchment then use as you wish.
This cake lasts well in an airtight container for a few days. I have made it without the apple and it tastes just as good; it’s just not as moist and therefore doesn’t keep quite as well.