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, adding extra water if needed. Shape into a disk, wrap in plastic, and refrigerate for 30 minutes.
02 - Roll out pastry on lightly floured surface and line a 9-inch tart pan. Trim excess pastry and prick base with 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 8–10 minutes until golden. Cool slightly.
04 - Combine apples, sugar, lemon zest, juice, and water in 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. Stir in butter. Beat egg yolks and quickly stir into warm apple mixture to prevent scrambling. Spoon filling into prepared pastry case and smooth 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 pastry edge to seal. Bake at 350°F for 20–25 minutes until meringue is golden. Cool for at least 1 hour before slicing.