Step 1Heat the oil in a large pot over medium heat. Add the onions and cook until they're translucent, 5 to 6 minutes. Add the carrots and celery and cook for a few more minutes. Add the lentils, water, thyme and bay leaf. Simmer, stirring occasionally, until the lentils are soft and the soup has thickened, 1 to 1 1/2 hours. Add salt to taste.
Step 2Puree about 2 cups of the soup, return it to the pot and heat through before serving.