Have a large bowl of ice water ready for Step 2. Place a small bowl and beaters or a whisk in the freezer to chill for Step 3.

Place 1/4 cup water in a small bowl; sprinkle in gelatin and stir to blend. Set aside to soften. Whisk eggs, 1/2 cup sugar (or Splenda), zest and lemon juice in a medium nonreactive saucepan. Place over low heat and cook, whisking constantly, until the mixture thickens slightly and an instant-read thermometer registers 160 degrees F, 8 to 12 minutes. Remove from heat. Add the softened gelatin; whisk until blended. Transfer to a large bowl. Set the bowl over the bowl of ice water and stir gently with a rubber spatula just until the mixture starts to thicken slightly, 3 to 5 minutes. Remove from the ice bath and set aside, but stir occasionally.

Whip cream in the chilled bowl until soft peaks form.

Place reconstituted egg whites and cream of tartar in a large bowl. Beat with an electric mixer on low speed until frothy. Increase speed to high and beat until soft peaks form. Gradually add the remaining 1/2 cup sugar (or Splenda), beating until the meringue is stiff and glossy.

Whisk one-fourth of the meringue into the cooled lemon mixture. Fold in the remaining meringue with a whisk. Fold in the whipped cream with a rubber spatula. Divide the mixture among eight 2/3- to 3/4-cup dessert dishes or wineglasses. Cover and refrigerate until set, about 2 hours.

Working with gelatin: When making mousses that are set with gelatin, chill the base mixture quickly by setting it over a larger bowl of ice and water before folding in the beaten egg whites and whipped cream. This thickens the base enough to give it a similar consistency to the beaten egg whites and whipped cream, making for a homogeneous mixture and better volume. If the base starts to set too quickly, simply place it over a bowl of warm water and stir gently for a few seconds.

To make decorative swirls in the sauce, whisk 2 tablespoons nonfat vanilla yogurt and 2 tablespoons low-fat milk until smooth. Drop small dollops of the mixture onto the sauce and draw the tip of a skewer or knife through them.

Ingredient note: Pasteurized dried egg whites called Just Whites can be found with baking ingredients or in the natural-foods section of most supermarkets. Reconstitute them according to package directions. They are distributed by Deb-El Foods. For more information, call (908) 351-0330.

MAKE AHEAD TIP: Cover and refrigerate for up to 2 days.