Place potatoes in a saucepan; cover with water, and bring to a boil. Cover, reduce heat, and simmer 15 minutes or until tender; drain. Set potatoes aside, and keep warm.
Position knife blade in food processor bowl. Drop dillweed, flat-leaf parsley, green onions, and garlic halves through food chute with processor running; process 3 seconds or until minced. Add nonfat buttermilk, lemon juice, and pepper; process until smooth.
Cut potatoes in half. Combine potatoes, celery, and bell pepper in a large bowl. Add buttermilk mixture, tossing gently to coat.
