Heat the butter in a saucepan and gently fry the onion for five minutes until soft but not coloured.
Add the garlic and fry for one minute.
Add the chopped tomato, lentils and chicken stock and bring to the boil. Reduce the heat and simmer for 12-15 minutes, or until the lentils are tender. Allow to cool slightly.
Pour the soup into a blender, add the spinach and herbs and blend until smooth. Season, to taste, with salt and freshly ground black pepper. Reheat the soup if necessary.
To serve, pour the soup into a serving bowl and swirl in a spoonful of double cream.
