Sauté onion, celery and garlic in butter in medium sized saucepan until tender but not browned, stirring often. Stir in curry powder. Mix in flour until smooth, cook 1 minute stirring constantly. Remove from stove. Add water to chicken broth to make total of 4 cups. Gradually stir into curry mixture until smooth. Stir in pepper. Cook over medium heat, stirring constantly until mixture thickens and boils. Cool slightly. Place broth mixture and peanut butter in blender or processor. Whirl until smooth. Return to saucepan. Bring to boiling, lower heat, cover and simmer 10 minutes. Place yogurt in small bowl. Gradually stir in some hot soup. Return to saucepan. Gently heat just to serving temperature. DO NOT BOIL. Stir in lemon juice. Ladle into bowls. Sprinkle with chopped peanuts and parsley.