01 - Preheat oven to 350°F. Lightly grease a 9-inch tart tin with butter.
02 - In a large bowl, rub flour, salt, and cold butter together until mixture resembles coarse breadcrumbs. Stir in sugar. Add egg yolk and mix well. Gradually add enough cold water to bring dough together into a ball. Wrap in plastic and refrigerate for 20 minutes.
03 - Roll out chilled pastry on a floured surface and carefully transfer to tart pan. Trim excess dough from edges. Prick base all over with a fork. Line with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes, then remove weights and parchment and bake 5 minutes longer until crust is lightly golden.
04 - In a saucepan, combine sliced apples, sugar, water, and lemon juice. Cover and cook over medium heat for 10-12 minutes until apples are completely soft and breaking down. Remove from heat and mash into a smooth purée. Let cool slightly, then stir in egg yolks and cinnamon.
05 - Spread apple mixture evenly into the pre-baked tart shell and smooth the top surface.
06 - Using an electric mixer or whisk, beat egg whites with cream of tartar or lemon juice until soft peaks form. Gradually add sugar while continuing to beat until stiff, glossy peaks form and sugar is completely dissolved.
07 - Spread or pipe meringue over apple filling, ensuring it completely covers filling and seals to edges of pastry. Bake for 25-30 minutes until meringue is crisp and golden brown.
08 - Allow tart to cool completely at room temperature before slicing and serving. For best results, let it set for at least 1 hour.