Heat oil in a large stock pot over medium heat. Add carrots, celery, onion and garlic and cook, stirring until tender. Stir in tomatoes and cook for a minute. Add stock, lentils, uncooked brown rice (if you’re adding it here), thyme and 2 teaspoons salt.Bring to a boil, reduce heat to a simmer and cook until lentils are tender, about 45 minutes.Pull out the thyme sprigs and discard. Add greens and stir until wilted.Season to taste with salt and pepper. If you didn’t add raw brown rice to the pot with your lentils, add the cooked brown rice to the pot now or put a little in each bowl and ladle the soup on top.