A pastry shell filled with lemon curd goes back to European kitchens, but topping it with meringue is an American tradition dating from the 1800s. Take care to measure out all meringue ingredients in advance, because you must make it and spoon it on top of the pie while the filling is still hot.
Beating in the Sugar
Beat the egg whites until the peaks bend over in soft curls when the beaters are lifted from the bowl. Sprinkle in sugar, 1 tablespoon at a time, beating constantly.
Testing the Peaks
The meringue is ready when the egg white-sugar mixture forms glossy peaks that stand up straight when the beaters are removed.
Spreading on the Pie
Spread the meringue with an icing knife or rubber spatula, sealing it completely to the edges. Swirl to make decorative peaks.