Start by zesting two medium lemons to gather about 2 teaspoons of zest. Juice the lemons until you have half a cup of juice. Set aside.

In a large bowl, whip 1 cup of heavy cream on medium speed until soft peaks form (about 3-5 minutes).

In a separate bowl, combine 2 egg whites with 1/2 cup of sugar and whip on high speed until stiff peaks develop (4-6 minutes).

Fold the lemon zest and juice into the whipped cream. Gently fold in the whipped egg whites until no streaks remain.

Transfer the mousse to serving dishes. Cover and chill in the refrigerator for at least four hours, preferably overnight.

Serve chilled, optionally garnished with lemon zest or fresh berries.