01 - Preheat oven to 350°F. Grease a 9-inch tart tin with removable base.
02 - In a bowl, rub together flour, butter, sugar, and salt until mixture resembles breadcrumbs. Add egg yolk and enough cold water to form a dough. Knead briefly, wrap, and chill for 20 minutes.
03 - Roll out pastry to fit the tart tin. Line the tin, trim edges, prick base with a fork, and chill for 10 minutes.
04 - Line pastry with parchment and baking weights; bake for 15 minutes. Remove weights and bake 5 more minutes until lightly golden. Cool.
05 - In a saucepan, combine apples, sugar, lemon juice, and zest. Cook over medium heat, stirring, until apples are soft and pulpy for 10-12 minutes. Remove from heat, mash gently, and stir in butter. Cool 10 minutes, then whisk in egg yolks. Spread filling in pastry shell.
06 - In a clean bowl, whisk egg whites until soft peaks form. Gradually add sugar and cream of tartar, beating to stiff, glossy peaks. Beat in vanilla.
07 - Spoon or pipe meringue over apple filling, swirling decoratively.
08 - Bake for 20-25 minutes, until meringue is golden and crisp. Cool in tin. Serve warm or at room temperature.