2/3 cup plus 1/4 cup sugar 3 large egg whites 1/2 teaspoon cream of tartar 1 teaspoon unflavored gelatin 1 large egg 1/2 cup fresh lemon juice 1 Tablespoon grated lemon zest 1/4 cup whipping cream Lemon slices for garnish
In a saucepan, bring about 1 inch of water to a simmer.
In a heat-proof mixing bowl, large enough to fit over the saucepan, combine 2/3 cup sugar, egg whites, cream of tartar and 3 tablespoons of water. Set bowl over barely simmering water and beat with an electric mixer at low speed, moving beaters around bowl constantly, until an instant-read thermometer registers 140ºF. (This will take 3 to 5 minutes.) Increase mixer speed to high and continue beating over heat a full 3-1/2 minutes. Remove bowl from heat and beat meringue until cool, about 4 minutes. Set aside.
In a small bowl, add 3 tablespoons cold water, sprinkling in gelatin. Allow to stand for 2 minutes to soften. Melt softened gelatin in microwave or over simmering water. Set aside.
In another heat-proof bowl, large enough to fit over saucepan of simmering water, combine whole eggs, lemon juice, lemon zest and remaining 1/4 cup sugar. Set bowl over barely simmering water and whisk slowly and constantly until mixture thickens and reaches a temperature of 160ºF. Remove bowl from heat and whisk in melted gelatin. Let cool for 20 minutes.