Heat the olive oil over medium heat in a small saucepan. Add the onions, garlic and jalapeño peppers and cook, stirring occasionally, until very soft, about ten minutes. Do not brown.
Transfer the cooked onion mixture to a blender or food processor and add the black beans, salt, cumin, lime juice, water and cilantro. Blend until smooth (if you're using a blender, you may need to stop and stir a few times to help it out). If the dip is too thick, add more water or lime juice, little by little, to thin it out. Taste and adjust seasoning, then transfer to a serving bowl. Serve warm, cold or room temperature with tortilla chips.
*Most of the heat from the jalapeños is in the seeds and ribs, so leave those out. If you like a spicier dip, reserve the seeds and add them little by little when you're blending the dip.