This is a cake that comes from Alto Adige in far northern Italy—in fact, it's origins are from nearby Germany and Austria. You can usually find it plain, or simply topped with powdered sugar. I topped this cake with 2 mandarins, sliced thinly, cooked for a few minutes in about 1/2 cup of mandarin marmalade and a splash of water.
Diana Henry makes a version of the Austrian cake where she uses walnuts in place of the almonds, splits the batter into two tins (which you could easily do with this batter) and serves them stacked on top of each other with cream and blackberry jam in between. And there's nothing to stop you putting a layer of lemon buttercream (or any layer, above or in between) on this cake.
Zest the lemon and place the zest together with the sugar in a large bowl and rub together until the sugar is damp and fragrant.
Separate the eggs into two large bowls. Beat the yolks with the sugar and lemon zest until creamy and pale. Wash the beaters very well, then beat the whites until soft peaks form.
Juice the lemon and stir through the grated apple.
Place the whole almonds in a food processor and blitz until they resemble fine sand. Add to the egg yolk and sugar, along with the poppy seeds, grated apple (and splash of rum if using). Stir through the baking powder. Finally, fold in the egg whites.
Pour into a greased and lined 10 inch (25cm) cake tin and bake for 40 minutes or until the top is springy and golden brown. Let cool and decorate with one of the ideas in the notes.