Melt the butter in a large saucepan, add the bacon, onion, carrot and leek and cook gently for 5 minutes. Add the peas, herbs and stock or water and bring to the boil. Boil rapidly for 10 minutes. Reduce the heat, cover and simmer the soup for about 50 minutes, or until the peas are tender. Remove the pan from the heat and discard the bay leaf and thyme. Allow to cool slightly, then purée the soup in a food processor or blender.
Return the purée to the pan and stir in the milk. Bring to the boil, reduce the heat and cook gently for 1–2 minutes. Season to taste, then pour into six heated bowls and top with a little sour cream, a sprinkling of chopped fresh mint and some croutons.