Heat oven to 160ºc or 140ºc for a fan-forced oven.
Lightly grease and line the bottom of a 20cm spring form cake tin with baking paper.
Pour half the warmed honey onto the baking paper in the cake tin, arrange the thin apple slices on top (leave a gap between the apples and the side of the cake tin of about ½ cm) and then pour the remaining honey over the apples.
Tip the tin if necessary to spread the honey evenly.
Whisk the egg whites until soft peaks.
Put to one side, while you whisk in a second bowl the egg yolks until thick and creamy and then, still beating gradually add the sugar and continue whisking until really light and creamy.
Fold the lemon zest into this mixture and add the flour and salt, stirring to incorporate and then add the melted butter and yoghurt and continue to stir until all mixed well.
Finally add the whisked egg whites and with minimum stirring blend all together.
Pour careful on top of apple and honey in tin, smoothing the top if needed, but it will usually settle on its own.
Bake for around 50 mins. Golden on the top and skewer test, before taking out of the oven. Rest in the tin for 10 minutes, before decanting! It should come out easily!