Bloom the gelatin by sprinkling it over cold water in a small bowl; set aside for 5 minutes.

Whisk egg yolks, half of the granulated sugar, lemon juice, and lemon zest in a heatproof bowl set over a saucepan of simmering water until the mixture thickens.

Remove the bowl from heat, stir in the bloomed gelatin until fully dissolved, then let the mixture cool to room temperature.

In a separate large bowl, whip the heavy cream until soft peaks begin to form.

In another clean bowl, whisk the egg whites with a pinch of salt until foamy, then gradually add the remaining sugar and continue whisking until stiff, glossy peaks form.

Gently fold the cooled lemon-yolk mixture into the whipped cream until evenly combined.

Carefully fold the whipped egg whites into the lemon-cream mixture until no streaks of egg white remain and the mousse is light and airy.

Divide the mousse into individual serving glasses or a large bowl, then chill in the refrigerator for at least 2 hours, or until completely set.