Place the spinach, lettuce and spring onions in a blender. Finely chop the cucumber and rocket leaves and add along with the parsley and half the mint.
Pour in the sour cream and process until smooth, adding the stock a little at a time until you have a smooth, green soup. Season and pour into a bowl. Chill for at least three hours. Before serving, adjust the seasoning if necessary and sprinkle with a little more mint if desired.