Step 1 In a small bowl, drizzle the gelatin over the cold water to soak.
Step 2 Meanwhile, in a medium metal bowl, whisk together the egg yolks with 1/2 cup sugar, the salt, lemon juice and lemon zest. Place the bowl over a larger pot of simmering water and cook, stirring constantly, until the mixture is thickened to a custard-like consistency, about 8 minutes. Add the soaked gelatin and stir until dissolved. Remove from heat and place the mixture over an ice bath, stirring just until cooled to room temperature. Remove from the ice bath and set aside.
Step 3In the bowl of a stand mixer, or in a large bowl using an electric mixer, beat the egg whites until foamy. With the mixer running, drizzle the remaining 1/2 cup of sugar over the whites and continue beating until the whites form stiff peaks.
Step 4In a separate bowl, beat the heavy cream until stiff peaks form.
Step 5Mix approximately one-fourth of the beaten egg whites into the cooled lemon mixture to lighten it. Fold in the remaining egg whites, followed by the stiffly whipped cream, taking care not to deflate the filling as it is combined.
Step 6Turn the filling into the baked pie crust and refrigerate at least 4 hours to allow the filling to set up.