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 extra tablespoon water if needed. Shape into disk, wrap in plastic, and refrigerate 30 minutes.
02 - Roll pastry on lightly floured surface and line 9-inch tart pan. Trim excess pastry and prick base with fork. Refrigerate 15 minutes.
03 - Preheat oven to 375°F. Line pastry with parchment and fill with pie weights. Blind bake 15 minutes, remove weights and parchment, then bake 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 (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 at least 1 hour before slicing.