How to Make It
Put the plantains and water in a medium saucepan and bring to a boil. Cover and cook over low heat until the plantains are just tender when pierced with a fork, about 10 minutes. Uncover and cook over moderate heat until most of the water has been absorbed and the plantains are completely tender, 10 to 15 minutes longer; don't let the plantains burn. Using a potato masher, mash the plantains in the saucepan until a slightly chunky puree forms; add a few tablespoons of water if the mixture is too stiff.
In a mortar or using the side of a chef's knife, mash the jalapeños and garlic to a coarse paste. Melt the butter in a small skillet. Add the jalapeño paste and cook over moderately high heat until fragrant and just beginning to brown, about 2 minutes. Add the onion and cook until softened, about 5 minutes. Add the seasoning mixture to the mashed plantains and cook, stirring, until heated through. Season with salt and serve.