In a large saucepan, melt the butter and in it cook the celery and onion over moderately low heat, stirring, for 15 minutes.
For the last minute of cooking, add the paprika and stir. Add half the stock, or enough to cover the vegetables, the bay leaf, and the savory and bring to a boil.
Lower the heat and simmer, partially covered, for 20 minutes, or until the vegetables are tender.
Remove the bay leaf and purée the vegetables in a food processor or food mill.
Put the purée in a stainless steel saucepan and add the remaining stock and the tomatoes. Bring to a boil, lower the heat, and simmer for 5 minutes. Add salt and pepper to taste.
In a small bowl, blend the heavy cream and sour cream and beat in the egg yolks. Whisk in a cup of the hot soup. Whisk this mixture into the soup and simmer until it thickens, stirring constantly. Do not allow the soup to boil or the eggs will curdle.
Serve in heated bowls and garnish with the chopped fresh parsley.