Whisk together 140 grams (1/2 cup plus 3 tablespoons) of the sugar with the cornstarch in a medium bowl until smooth. Whisk in the egg yolks until very smooth and thick. Stash near the cooktop. Place a fine-mesh sieve over a second medium bowl; stash this next to the cooktop as well.
Combine the water, 1/4 teaspoon of the salt, and another 140 grams (1/2 cup plus 3 tablespoons) more sugar in a medium saucepan. Place over medium-high heat and cook, whisking frequently, until the sugar dissolves and the mixture is very steamy, forms small bubbles around the edges of the pan, and registers 160°F, 2 to 3 minutes. Whisk in the lemon juice and lemon zest, then remove from the heat.
While whisking constantly, slowly ladle in about one-third of the sugar mixture into the egg mixture to temper the yolks. (The mixture will be quite thick at first.) Whisk this egg mixture back into the saucepan and place over medium-low heat. Cook, whisking constantly, until beginning to thicken and bubble, 3 to 5 minutes. Continue to cook until the mixture begins to turn translucent and registers 200°F, 1 to 2 more minutes. Immediately pour through the prepared sieve into the clean bowl to strain out any rouge bits of egg. Whisk in the butter, a few cubes at a time, until smooth. Pour into the cooled pie shell. Let cool completely before making the meringue, 2 to 3 hours.
When the filling has cooled and set, prepare the meringue topping: Place a folded kitchen towel in the bottom of an 11-inch straight-sided sauté pan or large skillet. Fill the pan with 1 inch of water. Place over medium-high heat and bring to a bare simmer, about 190°F. You should see steam and tiny bubbles, but the bubbles shouldn’t be breaking rapidly on the surface.
Whisk together the egg whites, cream of tartar, remaining 125 grams (1/2 cup plus 2 tablespoons) sugar, and remaining 1/4 teaspoon salt in the bowl of a stand mixer until smooth and foamy, about 30 seconds. Place the stand mixer bowl on the kitchen towel in the hot water, tilting the bowl as needed so the egg mixture is sitting in the water. Reduce the heat to medium and let heat, whisking constantly, until the mixture reaches 165 degrees F, 5 to 10 minutes.
Place the bowl in the stand mixer. Fit the mixture with the whisk attachment, then beat on high speed until soft peaks form, about 1 minute. Reduce the speed to medium and continue to beat until stiff peaks form, 3 to 5 minutes more.
Top the pie with the meringue, making sure to spread the meringue all of the way to the edges of the crust. If desired, use a spoon to create swirls and waves inthe meringue. Use a culinary or propane gas torch to brown the top, if desired. But who doesn't desire torching a meringue? Serve immediately or refrigerate up to overnight before serving.