Fry the potato and onion gently in the oil for a few minutes without browning. Wash, trim and roughly chop the green vegetables as appropriate. Any tough ones, such as fennel or celery, should be put on to cook at this stage. Add about ¼ pt / 140 ml stock, and cook until the vegetables are soft. Add the rest of the greens and half the herbs, and cook until the greens are done. Allow to cool slightly, make a purée in a blender or food processor, sieve, and stir in the rest of the stock. Finely chop the remaining herbs, and stir these in. Chill until ready to serve. Just before serving, season to taste, and, if using, stir in the yoghurt or single cream.