Before you start on the filling, please make the crust (see directions below) or have a store bought pre-baked or no-cook crust ready--you will need the crust ready as soon as the filling is cooked. It is important to assemble the pie as soon as the lemon filling is cooked and while it is still hot, so that the meringue topping is set onto hot filling and then baked to cook completely.
Grate or zest two lemons (yellow only, avoid the bitter white pith entirely) and mix with 1 1/2 cups sugar and 6 level tablespoons of cornstarch in a small bowl. Mix well.
Juice the zested lemons into a bowl or cup and strain the seeds out. Set aside for a moment.
In a separate set of bowls, separate three eggs with yolks in one medium bowl and whites in the other. Set the whites aside in the fridge, as you will whip them later into meringue.
Beat the egg yolks well then add them to the dry mix of sugar and constarch and zest along with the juice of the zested lemons, plus half a cup of additional lemon juice, two cups of BOILING water and "a piece of butter the size of a walnut" in the old recipe is 2 Tablespoons. Mix well.
This lemon mixture needs to cook in a double boiler so that it stays a nice light lemony yellow and does not darken as it cooks. If you do not have a double boiler, you can improvise like I did: pour the mixture into a smaller saucepan, and set that saucepan into a larger one that is partially filled with water. I set it on a metal steamer in a larger pan filled with water.
While the mixture is cooking, begin the meringue topping by beating the egg whites. I use a stand mixer and whip the egg whites while I'm cooking the filling. When the egg whites are opaque white, add the sugar and continue to beat until they are stiff.
Heat the lemon mixture over medium high on a double boiler. Once the water boils, cook, stirring frequently. It should thicken to a firm mousse or pudding texture within just a few minutes, Remove from heat and pour into a pre-baked crust.