Begin by zesting the two large lemons using a fine grater, collecting about 2 tablespoons of zest. Juice the lemons until you have about one cup of fresh lemon juice, strain to remove seeds, then set both aside.

In a chilled mixing bowl, pour in 1 cup of heavy whipping cream and beat on medium-high speed for 3-5 minutes until soft peaks form, being careful not to over-whip.

Whisk together 3 egg whites and 1/2 cup of granulated sugar in a heatproof bowl over a simmering pot of water. Continuously whisk for about 3-5 minutes until the sugar dissolves and mixture feels warm.

Remove from heat and beat the warm egg mixture on high speed until stiff peaks form, around 3-4 minutes.

Gently fold the whipped cream into the egg mixture, then add in reserved lemon juice and zest, folding until just combined.

Spoon mousse into serving glasses or bowls, cover with plastic wrap, and refrigerate for at least 4 hours.

Once chilled, garnish with additional lemon zest or fresh berries, and serve immediately.