1. Prepare the Lemon Base:

1. Fill a large saucepan with a few inches of water and bring it to a simmer over medium heat.
2. In a heat-proof bowl, whisk together 3 whole eggs, 3 egg yolks, 1 cup sugar, lemon zest, lemon juice, and a pinch of salt.
3. Place the bowl over the simmering water (double boiler method) and cook, stirring constantly, for about 15 minutes, until the mixture thickens slightly.
4. Remove from heat, let cool for 15 minutes, then cover with plastic wrap and refrigerate for at least 1-2 hours until fully chilled.

2. Whip the Egg Whites:

1. In a mixing bowl, beat the reserved 3 egg whites on high speed for 1 minute.
2. Add the remaining 2 tablespoons of sugar and continue to beat until stiff peaks form.
3. Gently fold the beaten egg whites into the chilled lemon mixture.

3. Whip the Cream and Combine:

1. In a separate bowl, beat the heavy whipping cream on high speed until stiff peaks form.
2. Carefully fold the whipped cream into the lemon mixture until smooth and fully incorporated.

4. Chill and Serve:

1. Refrigerate the mousse for at least 30 minutes before serving.
2. Serve chilled, garnished with fresh berries or whipped cream.