01 - Preheat oven to 350°F. Generously grease a 9-inch tart tin with removable base.
02 - Rub together flour, butter, sugar, and salt in a bowl until mixture resembles coarse breadcrumbs. Add egg yolk and enough ice water to form a dough. Knead briefly, wrap in plastic, and refrigerate for 20 minutes.
03 - Roll out pastry on a floured surface to fit the tart tin. Carefully transfer dough to tin, pressing into edges. Trim excess dough, prick base all over with a fork, and chill for 10 minutes.
04 - Line pastry shell with parchment paper and fill with baking weights or dried beans. Bake for 15 minutes. Remove weights and parchment, then bake 5 additional minutes until lightly golden. Let cool completely.
05 - Combine sliced apples, sugar, lemon juice, and zest in a saucepan. Cook over medium heat, stirring frequently, until apples break down into a soft pulp (10-12 minutes). Remove from heat, mash slightly, and stir in butter until melted. Cool 10 minutes, then whisk in egg yolks until smooth.
06 - Spread cooled apple filling evenly into the baked pastry shell.
07 - Whisk egg whites in a clean, grease-free bowl until soft peaks form. Gradually add sugar and cream of tartar, beating continuously until stiff, glossy peaks form. Fold in vanilla extract.
08 - Spoon meringue over apple filling, spreading to seal edges. Create decorative swirls with the back of a spoon or pipe for a finished look.
09 - Bake for 20-25 minutes until meringue is golden brown and crisp to the touch. Cool completely in the tin before removing. Serve warm or at room temperature.