Prepare the Lemon Mixture: In a mixing bowl, combine the lemon juice, lemon zest, and granulated sugar. Stir until the sugar is dissolved. Set aside.

Whip the Cream: In a separate bowl, whip the heavy cream until soft peaks form. Be careful not to overwhip; you want it to be fluffy.

Beat Egg Whites: In another bowl, beat the egg whites with salt until stiff peaks form. This will give the mousse its light and airy texture.

Combine Mixtures: Gently fold the lemon mixture into the whipped cream until combined. Then, carefully fold in the beaten egg whites, taking care not to deflate the mixture.

Chill the Mousse: Spoon the lemon mousse into serving dishes or cups. Cover and refrigerate for at least 2 hours to set.

Serve: Once set, garnish with whipped cream, lemon slices, or fresh berries if desired.