01 - Preheat oven to 350°F. Grease a 9-inch tart tin with removable base thoroughly.
02 - Rub chilled butter into flour, sugar, and salt until mixture resembles coarse breadcrumbs. Add egg yolk and 2 tablespoons cold water, mixing until dough forms. Add additional water only if necessary to bind. Wrap dough in plastic and refrigerate for 20 minutes.
03 - Roll out chilled pastry on lightly floured surface to 1/8-inch thickness. Carefully line prepared tart tin, trim excess pastry from edges. Prick base all over with fork.
04 - Line pastry with parchment paper and fill with baking beans or pie weights. Blind bake for 15 minutes. Remove paper and weights, then bake additional 5 minutes until bottom is lightly golden.
05 - Combine sliced apples, sugar, lemon zest, lemon juice, cinnamon, and butter in saucepan. Cook over medium heat, stirring frequently, until apples are tender but still hold shape, approximately 10 minutes. Remove from heat and allow to cool slightly.
06 - Whisk egg yolks into slightly cooled apple mixture until thoroughly combined. Spoon apple filling evenly into blind-baked pastry case.
07 - In clean, dry bowl, whisk egg whites with cream of tartar until soft peaks form. Gradually add caster sugar while continuing to whisk, until stiff, glossy peaks develop. Fold in vanilla extract.
08 - Spoon or pipe meringue over apple filling, creating decorative peaks and ensuring complete coverage to edges. Bake for 20-25 minutes until meringue is golden brown with crisp exterior.
09 - Allow tart to cool completely in tin before removing. Serve slightly warm or at room temperature. For best slicing results, use sharp knife dipped in hot water.