Preheat oven to 160°C fan-forced. Generously grease a baking dish with butter.
Whisk eggs and caster sugar.
Add flour, cornflour and baking powder. Mix until smooth.
Gradually whisk in milk (and cream if using), melted butter, vanilla, salt and lemon zest until smooth.
Add the sliced apples to the custard mixture and stir until evenly coated.
Pour the mixture into the prepared baking dish and sprinkle the top with 1 tbsp sugar.
For a shallow baking dish, bake for approximately 45 minutes. For a deep baking dish, bake for approximately 1 hour and 10 minutes. If using a deep dish, cover loosely with aluminium foil during baking and remove the foil for the final 10 minutes to allow the top to brown.
Insert a skewer into the centre. It should come out without watery custard. The centre should be set, although it may still have a slight custardy wobble and as it cools it firms up.
Allow to cool slightly before serving.
