01 - Preheat oven to 350°F. Grease a 9-inch tart tin with butter.
02 - In a large bowl, rub the flour, salt, and cold butter together until the mixture resembles coarse breadcrumbs. Add the sugar. Mix in the egg yolk and enough cold water (2–3 tbsp) to bring the dough together. Wrap in plastic wrap and refrigerate for 20 minutes.
03 - Roll out the chilled pastry and line the prepared tart tin. Trim any excess overhang. Prick the base with a fork. Line with parchment paper and fill with baking beans or pie weights. Blind bake for 15 minutes, then remove beans and parchment and bake 5 minutes more until the crust is lightly golden.
04 - In a saucepan, combine apples, sugar, water, and lemon juice. Cover and cook over medium heat until apples are completely soft (10–12 minutes). Remove from heat, mash the apples to a smooth purée, and let cool slightly. Stir in the egg yolks and cinnamon if using.
05 - Spoon the cooled apple mixture into the baked tart shell and spread evenly with the back of a spoon.
06 - Whisk egg whites with cream of tartar or lemon juice until soft peaks form. Gradually add the sugar while continuing to whisk until stiff, glossy peaks form that hold their shape.
07 - Spread or pipe the meringue over the apple filling, ensuring it completely covers the filling and seals to the edges of the pastry to prevent shrinking during baking.
08 - Bake for 25–30 minutes, or until the meringue is crisp and golden brown. Remove from oven and cool completely in the tin before serving.