Cut off and discard bottom of celery stalk; chop ribs and leaves. Heat oil in a soup kettle or large, heavy-bottomed saucepan and sauté onions until soft and limp, but not browned, stirring frequently. Add celery and cook, stirring frequently, until softened, about 5 minutes. Add just enough chicken stock to make celery float. Add bay leaf and thyme. Bring to a boil. Cover, reduce heat, and let simmer gently for about 2 hours, until celery is tender. Remove bay leaf. Process mixture in a blender or food processor in batches, adding the flour to one of the batches. (The blender makes a smoother soup.) Wipe pot with a paper towel. Return mixture to pot and bring to a boil. Reduce heat and let simmer for a minute or two to cook the flour. Gradually add enough cream or milk to make the desired consistency. Season to taste with salt and pepper. Spoon into heated soup bowls and garnish with a dab of drained yogurt or sour cream, if desired.
Makes 6 servings.