Place the lentils, carrots, parsnips, sweet potato, onion, and garlic into the slow cooker.
Add the vegetable broth, rosemary, bay leaf, salt, and pepper.
Stir everything together until the ingredients are evenly distributed.
Cover and cook on low for 7 to 8 hours until the lentils and vegetables are tender.
Remove the bay leaf and give the soup a quick stir before serving.