In a medium saucepan, cream 1/2 cup of unsalted butter and 1 cup of granulated sugar over low heat until fluffy and smooth.

Gradually incorporate 3 large eggs, mixing one at a time until fully blended.

Add in 1/2 cup of fresh lemon juice and 2 tablespoons of lemon zest, cooking over low heat for about 10 minutes, stirring constantly until thickened.

Remove from heat and let cool completely.

In a heatproof bowl, whisk together 2 large eggs, 1/2 cup of granulated sugar, 2 tablespoons of lemon zest, and 1/2 cup of fresh lemon juice.

Place the bowl over a pot of simmering water, ensuring the bowl doesn't touch the water.

Stir constantly for about 8-10 minutes, or until the mixture thickens and coats the back of a spoon.

Remove from heat and allow to cool.

In a clean mixing bowl, add 4 large egg whites and a pinch of salt.

Using an electric mixer, beat on medium speed until soft peaks form, about 3 minutes.

Gradually add 1/4 cup of sugar and continue beating until stiff peaks form.

Gently fold the prepared egg whites into the cooled lemon filling until just combined.

Once incorporated, delicately fold in the prepared lemon curd until the mousse is uniform and smooth.

Transfer the mousse into individual cups or a large glass serving dish.

Cover and refrigerate for at least 2-3 hours or until set.

Whip 1 cup of heavy cream with 2 tablespoons of powdered sugar until soft peaks form.

Top the mousse with whipped cream and garnish with fresh lemon slices before serving.