Preheat oven to 170°C degrees. Line and grease a round 20cm springform cake tin.
In a large bowl with a wooden spoon, beat together the egg yolks, lemon zest, lemon juice and ¼ cup sugar.
In another bowl whisk together the almond flour, coconut flour, baking powder, cinnamon and nutmeg. Add this flour mixture to the egg yolk mixture and beat together until well combined and forms a paste.
With your stand mixer and whisk attachment, start beating the egg whites on low speed gradually increasing the speed after 1 minute. Add the salt and vinegar and continue to beat as the egg whites increase in volume. Then gradually add the remaining ¼ cup sugar, a little at a time whilst continuing to beat until soft peaks form.
Gently fold the whipped egg white mixture into the almond mixture one large scoop at a time. As you do this, the mixture will transform from a thick paste to a lighter batter.
Scoop the batter into the prepared pan and bake in the oven for 30 minutes.
Remove from the oven and let cool for 5 minutes before running a knife around the edges to release the cake from the pan. Allow to cool completely and then gently place the cake on a serving dish.
For the meringue frosting: Bring a saucepan of water to a simmer. In a large heatproof bowl, whisk together the egg whites, sugar and ¼ cup cold water and then place the bowl over the saucepan of simmering water. Using a handheld electric mixer, beat egg whites for 12-15 minutes until stiff peaks form. Then remove bowl from heat and beat for a further 2 minutes until mixture is cooled.
Spread the meringue frosting onto the cake with a pallet knife lifting gently to form soft peaks. If desired, decorate with a slice of lemon.