In a large nonstick saucepan, heat the oil over medium heat. Add the onion and celery. Cook for 3 minutes or until crisp-tender. Add the potatoes, chicken broth, salt and pepper. Bring to a boil. Cover and reduce the heat. Simmer for 15 to 20 minutes or until the potatoes are very tender. Stir in the milk. Spoon half of the potato mixture with the liquid into a blender container and cover. Place a folded kitchen towel over the lid and hold in place. Blend the mixture for 30 to 60 seconds or until the desired texture is reached. If necessary, stop the machine and scrape the sides of the container. Pour the blended mixture into a heat-proof bowl. Repeat with the remaining potato mixture. Return all of the blended potato mixture to the saucepan. Fold in the sour cream. Cook over low heat until thoroughly heated (do not boil). Garnish each serving with freshly cracked pepper and fresh herb leaves, if desired.