In a small bowl, mix the gelatin with the cold water and let it stand for 3 minutes.
In a double boiler, whisk the egg yolks, 1/4 cup granulated sugar, lemon juice, and salt. Cook over simmering water, stirring constantly until thickened, about 6 minutes.
Remove from the heat, add the lemon zest and softened gelatin, stirring until the gelatin is completely dissolved. Set aside to cool to room temperature.
In a separate bowl, beat the egg whites until soft peaks start to form. Gradually add the remaining 1/4 cup granulated sugar and continue beating until stiff peaks form.
Fold the egg whites into the cooled lemon mixture gently but thoroughly.
Pour the filling into the prepared crust. Refrigerate until set, about 3 hours, or overnight.