Place the lentils, carrots, potatoes, parsnip, onion, and garlic into the slow cooker.
Add the vegetable broth, tomato paste, thyme, bay leaf, salt, and pepper. Stir until the tomato paste is mostly dissolved.
Cover and cook on low for 7 hours or on high for 3.5 hours until the lentils are tender and the vegetables are soft.
Remove the bay leaf. Taste and adjust salt or pepper if needed.
Ladle into bowls and top with fresh parsley before serving.