Cream together 1/2 cup unsalted butter and 1 cup granulated sugar in a medium bowl until light and fluffy. Gradually add 4 whole eggs, mixing well after each addition. Stir in zest of 2 lemons and juice, then cook on low heat until thick, about 8 minutes. Transfer to a bowl, cover, and refrigerate until cooled.

In a heatproof bowl over simmering water, whisk together 2 whole eggs, 2 egg yolks, 1/2 cup sugar, zest of 1 lemon, 1/3 cup lemon juice, and a pinch of salt. Continuously whisk until thick, about 12 minutes. Remove from heat and let cool before refrigerating.

Beat 3 egg whites in a clean bowl until soft peaks form, about 2-3 minutes. Gradually add 1/4 cup sugar, continuing to beat until stiff peaks form, about 3 more minutes.

Once the lemon filling and curd have cooled, gently fold the whipped egg whites into the filling. Then fold in the chilled lemon curd.

Transfer the mixture into a serving dish or cups, cover with plastic wrap, and refrigerate for at least 2-3 hours or until set.

Just before serving, whip 1 cup heavy whipping cream with 2 tablespoons powdered sugar until soft peaks form. Spoon over the mousse and garnish with lemon slices or zest.