Homemade Lemon Meringue Pie is a classic crowd pleaser with sweet lemon filling and fluffy, baked meringue on top.
In a small bowl, stir together the cornstarch and 1/2 cup water until the cornstarch dissolves. In a saucepan, whisk together the egg yolks, sugar, lemon juice, and 1 cup water until well blended. Whisk in the cornstarch mixture, place over medium heat, and bring to a boil while whisking constantly (about 8 minutes). Boil until the mixture thickens and looks clear, about 1 minute. Remove from the heat and stir in the lemon zest. Immediately pour the warm filling into the cooled pie crust.
In a large, heatproof bowl, combine the egg whites and sugar. Set the bowl in a pan of simmering water (not touching). Stir gently until the sugar has dissolved in the mixture is warm, 1 to 2 minutes. Remove the bowl from the pan and add the cream of tartar and salt. Using an electric mixer, beat the egg whites on high speed until stiff peaks form. Using a rubber spatula, spread the meringue evenly over the filling, mounting it toward the center and spreading it out to the edges to seal the crust. Use the back of a spoon to form swirls and peaks on the meringue. Bake until the meringue is lightly brown, 12 to 17 minutes. Transfer the pie to a wire rack and let cool for 1 hour. Refrigerate until cold, at least 5 hours or up to overnight. Serve cold.