Heat the oil in a large pot over medium heat. Add the onion and garlic and cook until softened.
Add the curry powder, cumin and coriander and cook for 1 minute.
Add the carrots, celery and potato and cook for 5 minutes.
Add the broth or water, lentils and tomatoes with juice. Bring to a boil then reduce heat to low. Simmer for 20 minutes or until vegetables are tender.