1. CREAM & SUGAR: Put your double cream, sugar and chopped rosemary in a large saucepan over a very low heat. Slowly bring to the boil, stirring regularly. Watch carefully – when the cream starts boiling, boil for 3 minutes then take off the heat. Allow to cool for a few minutes.
2. LEMON JUICE: While the cream is cooking and cooling, zest and juice your lemons into a small bowl. Once the cream has cooled slightly, stir your lemon juice and zest into the cream mixture and stir vigorously until all incorporated.
3. SIEVE: Get a large jug ready and sieve your lemon posset mixture into the jug, leaving the rosemary, lemon zest and pips in the sieve. Discard the bits left in the sieve.
4. COOL: Whilst the mixture is warm (before it starts thickening), pour your lemony posset into your serving glasses. Move them to the fridge and leave to cool and set for 3 hours or overnight. When serving, top with a light grating of lemon zest and a rosemary sprig.
TIP: If you’re mildly pedantic and you want to make sure everyone gets the same size portion, sieve your mixture into a measuring jug or into a jug on a weighing scale, then equally divide your portions. You will have roughly 700ml lemon posset in total.