Heat the olive oil into a medium sized soup pot over medium heat. Once the oil is warm, add the onion, carrot, celery and salt and cook until the onions are translucent, approximately 3-5 minutes. Add the lentils, tomatoes, broth, cumin and pepper and stir to combine. Increase the heat to high and bring to a boil. Reduce the heat to low, cover and cook at a low simmer until the lentils are tender, approximately 40 minutes. Using a stick blender, puree to your preferred consistency.