Melt butter in large saucepan. Add onions and saute until soft and translucent, 3-5 minutes. Add garlic and saute until fragrant, about 1 minute.
Place onions and garlic into slow cooker with celery, beef broth, chicken broth and barley. Cook on low 4-6 hours, until barley is tender.
Stir in buttermilk and dill. Continue cooking until heated through, about 30 minutes. Remove and discard celery. Serve with additional dill sprinkled on top.