Finely grate the zest of both lemons, then juice 1 to 2 lemon halves until you have 4 teaspoons of lemon juice.
In a large bowl or the bowl of a stand mixer, whip the cream and mascarpone cheese at medium speed until smooth and soft peaks form, about 3 minutes.
Add the powdered sugar, lemon zest, and lemon juice and continue whipping until the mixture thickens, not quite to stiff peaks, about 2 minutes.
Spoon the mousse into a large serving bowl or individual bowls, cups, or ramekins.
For the best texture, cover and chill the mousse in the fridge for at least 1 hour and up to 3 days before serving.
