01 - Preheat oven to 350°F. Grease a 9-inch tart tin with removable base thoroughly.
02 - In a mixing bowl, rub chilled butter into flour, sugar, and salt until mixture resembles coarse breadcrumbs. Add egg yolk and 2 tbsp cold water, mixing until dough forms. Add additional water only if needed 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 the prepared tart tin, trimming excess pastry from edges. Prick base all over with fork. Line with parchment paper and fill with baking beans or pie weights.
04 - Bake pastry for 15 minutes. Remove baking beans and parchment paper. Return to oven and bake for 5 additional minutes until base is lightly golden. Set aside to cool slightly.
05 - In saucepan over medium heat, combine sliced apples, sugar, lemon zest, lemon juice, cinnamon, and butter. Cook, stirring occasionally, until apples are soft but still hold shape, approximately 10 minutes. Remove from heat and allow to cool for 5 minutes.
06 - Beat the 2 reserved egg yolks into the slightly cooled apple mixture, stirring quickly to prevent eggs from scrambling. Spread apple filling evenly into the pre-baked pastry shell.
07 - In completely clean, dry bowl, whisk egg whites with cream of tartar until soft peaks form. Gradually add caster sugar while continuing to whisk, beating until stiff, glossy peaks form and sugar has dissolved. Fold in vanilla extract.
08 - Spoon or pipe meringue over apple filling, starting from edges and working inward to create a seal. Use back of spoon to create decorative peaks and swirls.
09 - Return tart to oven and bake for 20-25 minutes until meringue peaks are lightly golden and crisp to the touch.
10 - Allow tart to cool completely in the tin before removing. Serve slightly warm or at room temperature. Slice into 8 portions.