In a medium saucepan, heat the oil over medium heat. Stir in the onion and, if using, the diced celery. Cook, stirring, until the vegetables are beginning to soften, about 6 minutes. If using celery base, stir it in now.
If starting with fresh tomatoes, stir them in to the onion mixture with the stock. Bring to a boil. Lower the heat and simmer for 15 minutes, or until the tomatoes are