Place potatoes, celery, carrot and onion in a 4-qt. slow cooker. In a small bowl, mix broth, parsley, 1 teaspoon salt, 1/2 teaspoon pepper and garlic; pour over vegetables. Cook, covered, on low 6-8 hours or until potatoes are tender., Transfer potatoes from slow cooker to a 15x10x1-in. pan; discard cooking liquid and vegetables (or save for another use). Using the bottom of a measuring cup, flatten potatoes slightly. Transfer to a large bowl; drizzle with butter. Sprinkle with remaining salt and pepper; toss to coat. Sprinkle with additional parsley.