Mix the cream, lemon zest and sugar in a saucepan over medium heat. Slowly bring to a boil, stirring until the sugar is dissolved. Reduce the heat and simmer for 3 minutes. Remove the pan from the heat and whisk in the lemon juice. Strain into a jug, pressing the zest in the sieve to extract as much flavor as possible; discard the zest.
Let settle for 5 minutes, then skim the froth from the surface of the cream (optional).
Pour equal amounts of the lemon posset into four glasses. Let cool. Cover with plastic wrap and refrigerate for 24 hours to set.
Garnish with raspberries and mint leaves just before serving.