Place the apple, cinnamon, lemon rind and juice in a non-metallic bowl and toss to combine. Set aside.
Place the eggs, extra yolks, vanilla and sugar in a heatproof bowl. Place over a saucepan of simmering water and, using a hand-held electric mixer, beat for 6–8 minutes or until thick and pale. Remove from the heat and allow to cool slightly.
Whisk the cream until stiff peaks form. Gently fold through the egg mixture until well combined. Fold through the apple mixture until well combined. Pour into a 2 litre-capacity metal tin and freeze for 6 hours or overnight.
Preheat oven to 180°C (355°F). To make the crumble, place the flour, sugar and butter in a bowl and mix well to combine. Spread out on a baking tray and bake for 15 minutes or until golden. Set aside to cool. Scoop semifreddo into waffle cones and coat with crumble to serve. Makes 2 litres.