Beat egg yolks until thick and lemon colored. Combine yolks, 1 cup sugar, lemon rind, juice, and salt in a saucepan; stir until smooth. Cook over medium heat, stirring constantly, 5 minutes or until thickened. Remove from heat;add gelatin, and stir until gelatin granules dissolve. Cool.
Beat egg whites ( at room temp) until foamy;gradually add 1/2 cup sugar, 1 tablespoon at a time, beating until soft peaks form. Fold egg whites and whipped cream into lemon mixture, pour into crust. Chill until set. Garnish with lemon slices.