Finely grate the zest of 2 lemons and juice them. Sift the powdered sugar into the heavy cream and beat with a mixer until it starts to thicken. Add the lemon zest, lemon juice, and limoncello. Whip until thick.
Spoon the mixture into 6 small dessert bowls or glasses and chill for at least 30 minutes. (Can be made up to 1 day ahead.)
Slice the remaining lemon into 6 slices and garnish the syllabub with lemon slices and mint sprigs.