01 - Combine flour, sugar, and salt in a bowl. Rub in cold butter until mixture resembles fine breadcrumbs. Add egg yolk and 2 tablespoons water, mixing until dough just comes together. Add an extra tablespoon of water if needed. Shape into a disk, wrap in plastic, and refrigerate for 30 minutes.
02 - Roll out pastry on a lightly floured surface and line a 9-inch tart pan. Trim excess pastry and prick the base with a fork. Refrigerate for 15 minutes.
03 - Preheat oven to 375°F. Line pastry with parchment and fill with pie weights. Blind bake for 15 minutes, remove weights and parchment, then bake for another 8–10 minutes until golden. Cool slightly.
04 - Combine apples, sugar, lemon zest, juice, and water in a saucepan. Cook over medium heat, stirring occasionally, until apples are very soft and most liquid has evaporated, about 10–12 minutes.
05 - Remove from heat and stir in butter. Beat egg yolks and quickly stir into the warm apple mixture to prevent scrambling. Spoon filling into the prepared pastry case and smooth the surface.
06 - Whisk egg whites and cream of tartar until soft peaks form. Gradually add sugar, whisking until stiff, glossy peaks form.
07 - Spread meringue over apple filling, swirling decoratively and ensuring it touches the pastry edge to seal. Bake at 350°F for 20–25 minutes until meringue is golden. Cool for at least 1 hour before slicing.