For the rosemary syrup
15g rosemary sprigs 2 tbsp caster sugar
Preheat the oven to 180°C/fan 160°C/gas 4. Grease a 23cm cake tin and line the base with parchment paper. Mix the sliced apples with the lemon juice.
Put the eggs and sugar in a bowl and whisk well until pale and fluffy. Sift half the flour, all the baking powder and the salt over the mixture and fold in. Gently add the melted butter, the rest of the flour, the chopped rosemary and then the apple slices. Mix well. Spoon into the prepared tin and bake for 40 minutes until a skewer inserted in the middle comes out clean.
Meanwhile, make the rosemary syrup. Wash the rosemary and put the small sprigs into a pan with the sugar and 5 tbsp water. Cook over a low heat for about 10 minutes until the sugar has turned into a clear syrup and the rosemary has crystallised.
Remove cake from the oven and leave to cool in the tin for about 5 minutes, then turn on to a serving plate and pour over the rosemary syrup. Keep the cake at room temperature, covered, and it will last for days. Sprinkle with vanilla sugar and the lemon zest before serving.