Put all the ingredients except the cream and rosemary into a pan and bring slowly to the boil. Pour into a bowl, cover and leave to infuse for 12–24 hours.
Strain into a large bowl and add the cream slowly, stirring all the time. Whisk until the mixture thickens and holds a soft peak on the whisk. It will take several minutes. Spoon into glasses, and refrigerate or put in a cool place u