Combine egg yolks, lemon rind and juice, and 1/2 cup sugar in top of a double boiler, mixing well. Bring water in bottom of double boiler to a boil. Cook, stirring constantly, until mixture thickens. Soften gelatin in cold water; add to lemon mixture, stirring well. Cool.
Beat egg whites (at room temperature) until foamy. Gradually add salt and remaining 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold egg whites into lemon mixture. Pour filling into pastry shell. Chill. Spread whipped cream over pie before serving.