Heat oven to 350F/180C.
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.
