Preheat your oven to 180°C and start with your pastry, rub the butter and plain flour until you have fine breadcrumbs with a few larger flakes, stir through caster sugar, then mix in the egg yolk and just enough cold water to bring it together into a soft dough.
Flatten, wrap, and chill for at least 30 minutes. Roll to about 3–4mm thick and use it to line a large, deep 23–25cm fluted pie dish (around 4–5cm deep).
Press it well into the edges, then chill again for 15–20 minutes before blind baking with baking paper and weights for 15 minutes, removing the weights and baking for a further 10–15 minutes until properly golden and cooked through.
Meanwhile, peel, core and chop the apples and cook them gently with a splash of water, the juice of half a lemon and 100–120g caster sugar until completely soft and thick, then allow to cool slightly before stirring in 3 egg yolks.
Spoon this into your baked pastry case and return to the oven for 10–15 minutes until just set.
Whisk the egg whites to soft peaks, gradually adding 75g caster sugar until glossy, then pile generously over the apple, creating soft peaks and swirls.
Bake at 160–170°C for 20–25 minutes until lightly golden and crisp on top.
Leave to sit for a short while before serving so the layers settle.
