@ossobuco,
OK! Grigson called it Margaret Costa's Green Soup -
http://able2know.org/topic/167429-7#post-4504340
from that post -----
from the unitynorthchurch website, instead of my copying my book -
MARGARET COSTA'S GREEN SOUP
(From Jane Grigson's Vegetable Book)
4 medium potatoes, peeled and diced
1 medium onion, chopped
4 cups of vegetable or chicken stock
salt, pepper, grated nutmeg
pinch of sugar (optional)
2 handfuls of sorrel, washed and de-ribbed
Cook onion and potatoes in stock. Add seasonings. Bring to a boil, turn down to a simmer and cook until the potatoes are done. Puree the soup in the blender with the sorrel leaves, until smooth and bright green. Return to the pan and check seasonings. Do not boil the soup after the sorrel has been added - sorrel will become dark green and lose some of its flavor. This is the basic soup.
Toppings and Options:
You can add 1/4 cup of heavy (whipping) cream. Or a dollop of plain yogurt or sour cream. Sprinkle with chives. Serve it hot or cold. It will thicken a bit when cold. Serve with bread cubes, croutons, or garlic toast.
(osso - I never added cream, etc.)