Heat the oil in a large soup pot over medium heat. Add the carrots, celery, onion, and garlic and cook, stirring often, for 5 to 8 minutes, until the vegetables are tender.
Add the broth, lentils, and thyme. Bring the soup to a boil. Cover, reduce the heat to a simmer, and cook for 20 minutes, or until the lentils are tender.
Add the kale and cook for 3 to 4 more minutes to wilt the kale. Season with salt and pepper.