Create a double boiler by setting a heatproof bowl over a saucepan with about an inch of simmering water. Ensure the bottom of the bowl does not touch the water.
In the heatproof bowl, whisk together the granular erythritol, lemon zest, lemon juice, and egg yolks.
Continue whisking constantly over the simmering water for about 6 minutes, until the sweetener has fully dissolved and the mixture has thickened enough to coat the back of a spoon.
Remove the bowl from the heat and allow the lemon mixture to cool for 5-10 minutes.
Whisk in the softened mascarpone and sour cream until smooth.
Place the bowl in the refrigerator to chill while you prepare the whipped cream.
To make the whipped cream, combine the cold heavy cream, powdered erythritol, and vanilla extract in a large mixing bowl.
Using an electric mixer, beat on high speed until light, fluffy, stiff peaks form.
Gently fold 1½ cups of the finished whipped cream into the chilled lemon mixture until just combined. Be careful not to overmix, as this will deflate the mousse.
Spoon the lemon mousse evenly into 6 serving glasses or ramekins.
Top with the remaining whipped cream, a sprinkle of extra lemon zest, and garnish with slivered almonds and fresh mint, if desired.
Chill for at least 30 minutes before serving.
When combining the whipped cream and lemon base, use a gentle folding motion with a spatula.
Cut down the middle, scrape along the bottom, and 'fold' it over. This preserves the airy texture.
