01 - Preheat oven to 350°F. Grease a 9-inch tart tin with butter.
02 - Rub flour, salt, and cold butter together in a large bowl until mixture resembles coarse breadcrumbs. Stir in sugar. Mix in egg yolk and enough cold water (2–3 tbsp) to form a dough. Wrap in plastic and refrigerate 20 minutes.
03 - Roll out chilled pastry and press into tart tin. Trim excess dough. Prick base with fork. Line with parchment paper, fill with baking beans or dried rice. Bake 15 minutes, remove beans and paper, bake 5 more minutes until lightly golden.
04 - Combine sliced apples, sugar, water, and lemon juice in a saucepan. Cover and cook over medium heat until apples are completely soft, 10–12 minutes. Remove from heat, mash to smooth purée, and cool slightly. Stir in egg yolks and cinnamon if using.
05 - Spread apple mixture evenly into the baked pastry crust.
06 - Whisk egg whites with cream of tartar or lemon juice until soft peaks form. Gradually add sugar while whisking until stiff, glossy peaks form.
07 - Spread or pipe meringue over apple filling, sealing completely to edges. Bake 25–30 minutes until meringue is crisp and golden brown.
08 - Allow tart to cool completely at room temperature before slicing and serving.