Preheat oven to 180ºC (355ºF) and grease a 23cm cake tin and line it with baking paper. I used a spring form tin, but a regular cake tin will do.
Sift the flour and icing sugar into a bowl and add the almond meal and lemon rind. /mix well to combine.
Add the egg whites, butter and olive oil and mix well to combine. The batter will look incredibly moist, but don't worry, this is how it should be.
Pour into the prepared cake tine and bake for 20–30 minutes (the time will vary depending on your oven but I suggest checking after 20 minutes) or until cooked when tested with a skewer. Allow to cool in the tin for 10 minutes, then turn out onto a plate.
To make the candied lemon slices place the sugar and water in a saucepan over medium heat and stir to dissolve the sugar.
Add the lemon slices and bring to the boil and cook for 12–15 minutes or until the syrup has thickened and the lemons are translucent.
Scoop the lemon slices out of the syrup onto a plate and pour the syrup over the cake and top with the lemon. Allow to cool before serving.