Place carrots and onion in a 4-qt. slow cooker; top with potatoes. Drizzle with melted butter; sprinkle with salt and pepper. Cook, covered, on low until vegetables are tender, 4-5 hours., Add peas to slow cooker. Cook, covered, on high until heated through, 10-15 minutes. Stir in lemon zest. Sprinkle with chives.