Bring the sugar to the boil with the specified amount of water, star anise and cinnamon sticks. As soon as the sugar has completely dissolved, add 700 g of the pitted and halved plums and simmer for five minutes. Remove the spices, puree the rest with a hand blender and pass through a fine sieve. Season to taste with lemon juice.
Cut the remaining plums into strips and add to the soup. Chill for a few hours and serve with a scoop of ice cream.