Slow Cooker Vegetable Barley Soup

Tie the thyme together with a piece of thread or butcher's twine. In a 5- to 6-quart slow cooker, combine the parsnips, celery, garlic, onion, rutabaga, thyme, barley, 8 cups water, 1 teaspoon salt, and 1/4 teaspoon pepper. Cook, covered, until the barley is cooked through and the vegetables are tender, 6 to 7 hours on low or 4 to 5 hours on high (if cooking on high, use only 6 cups of water). When the soup has 10 minutes left to cook, remove the thyme. If the slow cooker is on the low setting, turn it to high. Add the chard and cook, covered, until tender, 5 to 7 minutes. Serve with Parmesan, if desired.