Start by combining all-purpose flour, powdered sugar, and salt in a food processor. Add cold, cubed unsalted butter and pulse until the mixture resembles coarse crumbs, about 15-20 seconds. Incorporate the egg yolk and cold water, pulsing until the dough just comes together. Form the dough into a disc, wrap in plastic wrap, and refrigerate for at least 2 hours to chill.

Once the dough is chilled, lightly flour your work surface and roll it out into a 12-inch circle, about 1/8 inch thick. Carefully transfer the rolled dough to a 9-inch pie dish, gently pressing it into the sides and bottom. Use a fork to perforate the bottom and freeze for 30 minutes to firm up the crust before baking.

Preheat your oven to 350°F (175°C). Line the chilled crust with parchment paper, filling it with baking beans or rice for weight. Bake in the preheated oven for about 35 minutes, or until the edges are golden brown. Remove the weights and parchment, then cool completely on a wire rack before filling it with lemon curd.

In a mixing bowl, whisk together the large egg yolks and cornstarch until smooth. In a saucepan over medium heat, combine water, freshly squeezed lemon juice, granulated sugar, salt, and lemon zest, stirring until dissolved. Gradually whisk the hot mixture into the egg yolks, then return it to the saucepan. Cook over medium heat, stirring constantly, until thickened—5-7 minutes. Remove from heat, stir in the butter until melted, then pour the filling into the cooled crust. Chill for at least 8 hours to set.

For the meringue, set up a double boiler by simmering water in a pot. In a heatproof bowl, combine egg whites, granulated sugar, and cream of tartar. Whisk constantly over simmering water until the sugar dissolves, about 4-5 minutes. Once dissolved, transfer to an electric mixer and beat on high speed until stiff peaks form, approximately 5-7 minutes.

Spread the fluffy meringue over the chilled lemon filling, creating peaks for added texture. Use a kitchen torch or place it under the broiler for 1-2 minutes to lightly toast the meringue, watching closely to prevent burning. Refrigerate the completed Lemon Meringue Tart until ready to serve, storing any leftovers in an airtight container for up to 3 days.