Heat oil in a large pot over medium heat and add onion, bay leaves, and salt. Stir occasionally until onions are softened, 4–5 minutes. Add celery, carrots, and green pepper and continue cooking, stirring occasionally, until all vegetables are softened, 7–8 minutes. Add tomato purée, bouillon cubes, and water and bring to a boil. Reduce heat to maintain a simmer and cook for about 15 minutes, or until vegetables are very tender.