Place the lentils, carrots, celery, onion, garlic, diced tomatoes with their juice, and vegetable broth into the slow cooker.
Add the thyme, cumin, and bay leaves. Season with salt and pepper.
Stir once to combine, then cover and cook on low for 7 to 8 hours until the lentils are soft.
Stir in the baby spinach during the final 10 minutes of cooking so it wilts evenly.
Remove the bay leaves before ladling the soup into bowls.