Heat oil in saucepot over medium heat. Add onion and celery; cook until softened stirring occasionally, 6 minutes. Add 4 cups water, asparagus stalks, beans, bouillon, tarragon, thyme and salt. Bring to simmer and cook covered, 20 minutes.
Meanwhile, bring a small saucepan of water to boil. Add asparagus tips and simmer 3 minutes or until tender. Drain and rinse under cold water. Reserve for garnish.
Blend soup and 1/3 cup sour cream in batches, in blender or processor, until smooth. Top each serving with asparagus tips and a small dollop of sour cream.