Place ½ cup of the vegetable broth in a large nonstick frying pan, add the onion, garlic, and jalapeno and cook over medium heat, stirring frequently, until vegetables are very soft, adding the rest of the vegetable broth, if necessary.
Add tomatoes, cilantro, and black pepper.
Cook, uncovered, over low heat, stirring occasionally, for 15 minutes. Meanwhile, drop the spinach into boiling water for 1 minute.