Preheat the oven to 350°F. Place the pie crust in a pan. Poke holes into the unbaked pie crust with a fork and bake it for about 20 minutes or until it’s lightly browned. Some people prefer to weigh it down with pie weights or beans as it may bubble a bit.
Meanwhile, in a pot stir together the sugar, flour, and hot water, and cook on medium until the sugar has dissolved, a couple of minutes. Beat the egg yolks with 1/4 cup cold water and add to the pot, then stir in the lemon juice, butter, and salt. Stir and cook until thick, about 5 minutes. Remove from the heat.
To make the meringue, beat the egg whites with the sugar and salt until they are smooth, light, and fluffy; they should have soft peaks like whipped cream. This should take anywhere from 5 to 10 minutes. (If you don’t have a stand mixer, a strong arm with a whisk or an eggbeater can accomplish this task, too. Please note that by hand it will take much longer than 10 minutes.) Stir in the sugar to the meringue.
Pour the lemon custard into the baked pie shell and top with the meringue. Bake until the peaks on the meringue are lightly browned, about 10 minutes.