1. Shell the peas you should have about 450g after shelling.
2. Melt the butter (or heat the oil) in a large saucepan and cook the bacon and spring onions until lightly golden. Add the lettuce and cook gently, stirring, until the lettuce has wilted.
3. Add the peas and chopped mint to the pan, stir in the chicken (or vegetable) stock and bring to the boil. Reduce the heat, cover and simmer for 20 minutes. Allow the soup to cool slightly, then transfer in batches to a liquidiser goblet and whizz until smooth. Pass the soup through a sieve into a large bowl.
4. Stir the cream into the soup, then chill in the fridge until required. Just before serving, stir in a little iced water to thin the soup to a suitable consistency. Season to taste and garnish with mint sprigs.