Rinse the beans well. Trim end and cut beans into ¼” long pieces.
Combine the beans, water, 3 tablespoons vinegar, onion, sugar, salt, garlic and tarragon stem in a sauce pan *. Bring liquid to a simmer, partially cover, and simmer for 15 minutes.
Blend together the flour and sour cream in a bowl. Stir one cup of the hot soup into the sour cream, then reverse the cream mixture into the soup. Return to a simmer and cook another 5 minutes before stirring in the remaining tablespoon of vinegar.
Chill thoroughly. Serve with garnish of fresh dill or mint.
* Use a coated aluminum, stainless steel, or ceramic covered cast-iron pan.
