In a large heatproof bowl, whisk together 3 whole eggs, 3 egg yolks, 1 cup sugar, lemon zest, lemon juice, and salt. Reserve the 3 egg whites separately.
Set the bowl over a saucepan of simmering water (double boiler). Stir constantly for about 15 minutes until thickened and reaching 150°F (66°C).
Remove from heat, cool for 15 minutes, then press plastic wrap onto the surface and refrigerate for at least 2 hours until fully cold.
Beat reserved egg whites on high for 1 minute. Add remaining 2 tbsp sugar and beat to stiff peaks.
Gently fold meringue into the cold custard in two or three additions.
In a clean bowl, beat heavy cream to stiff peaks. Fold into the mousse mixture. Fold in lemon curd now if using.
Spoon into serving glasses. Chill for at least 30 minutes. Top with whipped cream and lemon curd and serve cold.
