For the posset, bring the double cream and sugar to the boil in a pan and boil for 2-3 minutes. Remove from the heat and add the zest and juice of the lemons. Whisk vigorously and pour into glasses or moulds and place in the fridge for 2-3 hours.
For the strawberries, place a pan over a high heat and add the sugar. Form a caramel, then add in the strawberries and blueberries and cook for a minute. De-glaze the pan with crème de cassis and serve on top of the posset with a sprig of mint.