To a food processor add butter, icing sugar and flour and pulse until mixture forms fine crumbs. In a small bowl, beat egg yolk and water with a fork until combined, then add to processor and pulse until mixture starts to clump (add another 1 Tbsp water if needed).
Transfer pastry to a lightly floured surface and gently knead into a ball. Wrap in plastic wrap (or place in a ziplock bag) and chill for 30 minutes.
Preheat the oven to 185°C and line an oven tray with baking paper.
Peel, core and slice apples and place in a large bowl. Sprinkle with cinnamon, cornflour, sugar and zest and toss to mix.
Roll out pastry to about 40cm x 30cm to make a rough, rustic rectangle or oval, and transfer to lined tray. Arrange the cinnamon apples down the centre, then carefully fold the pastry over the filling, covering about half the apples. Brush pastry with egg-and-milk mixture and sprinkle with caster sugar.
Bake for 35-45 minutes or until the pastry is golden and the apples tender.
Serve warm with honey yoghurt cream.
To make the honey yoghurt cream, whisk together cream, vanilla yoghurt, runny honey and cinnamon until soft peaks form.