Creamy Lemon Mousse

Bring about 1 inch of water to a simmer in a saucepan. Combine 2/3 cup sugar, egg whites, cream of tartar and 3 tablespoons water in a heatproof mixing bowl large enough to fit over the saucepan. Set the bowl over the barely simmering water and beat with an electric mixer at low speed, moving the beaters around the bowl constantly, until an instant-read thermometer registers 140 degrees F, 3 to 5 minutes. Increase the mixer speed to high and continue beating over the heat a full 3 1/2 minutes. Remove the bowl from the heat and beat the meringue until cool, about 4 minutes. Set aside.

Place the remaining 3 tablespoons cold water in a small bowl. If using food coloring, add about 3 drops to the water. Sprinkle gelatin over the water and let stand for 2 minutes to soften. Microwave on High, uncovered, until the gelatin has completely dissolved but the liquid is not boiling, 10 to 20 seconds. (Alternatively, bring 1/2 inch water to a gentle simmer in a small skillet. Set the bowl with the gelatin mixture in the simmering water until the gelatin has dissolved completely.) Stir the mixture until smooth.

Combine whole eggs, lemon zest, lemon juice and the remaining 1/4 cup sugar in another heatproof bowl large enough to fit over the saucepan of simmering water. Set the bowl over the barely simmering water and whisk slowly and constantly until the mixture thickens and reaches a temperature of 160 degrees F. Remove the bowl from the heat and whisk in the melted gelatin. Let cool for 20 minutes.

Beat the cream in a chilled bowl until soft peaks form.

Whisk about one-fourth of the beaten egg whites into the cooled lemon mixture to lighten it. Add the remaining whites and use a whisk to incorporate them with a folding motion. Fold in the whipped cream with a rubber spatula.

Divide the mousse among 6 dessert dishes or stemmed glasses. Cover loosely and refrigerate until set, about 3 hours. Garnish with lemon slices before serving.