In a large pot heat 1 tablespoon of oil, add the bay leaf, allspice, onion, carrot and celery root and cook gently for 5 minutes stirring often.
Add the potatoes, half the stock, cover and bring to the boil then simmer for 10 minutes (or until the potatoes are fully cooked).
Add the sorrel, parsley, the rest of the stock, cover, bring to the boil again then simmer for 5 minutes.
Slowly pour in the beaten egg stirring all the time and cook for another minute. Remove from the heat, stir in the butter, season to taste and serve.
Notes
You can use both the leaves as well as the stem of the sorrel (chop the stem quite finely).
Sorrel will stay fresh stored in a plastic bag and refrigerated for up to 3 days.
If you prefer to use sorrel from a jar add 3-4 tablespoons or to taste. Polish delicatessens sell sorrel in jars.
I used 2 medium eggs but you can use 1 (large) if you prefer. Cook them in the soup, as I have done, or hard boil separately then add to individual portions.
Add a tablespoon of soy sauce for more depth of flavour if you like.
Be generous with the seasoning, especially the pepper. White pepper works well.
See the post for making sorrel soup with meat.
This soup tastes even better the next day. Refrigerate for up to 3 days.