Wash the apples well, then peel and core (compost or discard these), but keep the peels and add them to a small powerful blender and finely chop. Alternatively, finely chop by hand and reserve.
Using a sharp knife, cut the apples into 2 mm (1/16 in) thick slices, or use a mandoline.
Add the sliced apples to a large bowl with the sugar, cinnamon, vanilla, lemon juice, zest and cornflour and gently toss the apples until everything is properly combined. Leave to stand for 20 minutes to macerate – this will draw some moisture out of the apples.
Preheat the oven to 180°C fan (350°F/gas 4) with a baking rack in the bottom quarter of the oven.
Make or prepare your vegan flaky pastry of choice, and use to line a a 23 cm (9 in) tart case.
Roll out two-thirds of the pastry between two sheets of baking parchment into a large disc, about 30 cm (12 in) in diameter and about 3–4 mm (1⁄8–1/6 in) thick. Roll out the remaining pastry into a 25 cm (10 in) disc and 3 mm (1⁄8 in) thick.
You can remove the top sheet of baking parchment and use the bottom sheet to help flip it over into the dish, if you like. Gently press the larger disc of pastry into a 23 cm (9 in) pie or quiche dish with a tiny overhang.