Put the cream and sugar in a small saucepan. Heat until just boiling, then boil vigorously for 2½ mins, while stirring constantly. Turn off the heat, stir in the lime juice and most of the zest and divide between 2 small pots or glasses. Chill for at least 2 hrs (or overnight if you’re making ahead), until set.
To serve top each posset with a few raspberries and the remaining zest.