Place olive oil in a frying pan and put on medium heat. Add onions, carrots, celery and salt and allow them to cook until onions are translucent. Do not brown vegetables. In a large pot, combine onions mixture, lentils, tomatoes, vegetable broth, coriander, cumin and pepper. Stir to combine all ingredients and bring to a boil. Reduce heat, cover and simmer until lentils are tender (about forty minutes). To thicken the soup, allow soup to cool and puree 1/3 of it in food processor. Return pureed soup to remaining soup in pot and stir until combined. Serve hot.