Step 1Cut the tips off the plantains; cut each crosswise into 3 chunks. Place in a medium saucepan with water to cover by at least 1 inch. Bring to a boil over high heat. Cover the pot, reduce the heat to low, and cook until a knife easily pierces the skin and flesh, 20 to 30 minutes. If you are using less ripe plantains, the cooking time must be increased. Green ones may take 30 to 40 minutes and will not be as sweet as yellow ones. Drain well and peel, using a knife tip if necessary, to help detach the skin. Return to the pan and mash as smooth as possible with a wooden spoon or potato masher.
Step 2While the plantains are cooking, crush the garlic and jalapenos to a paste using a mortar and pestle, or puree in a mini-processor. Chop the onion.
Step 3In a small skillet, heat the lard to rippling over medium-heat. Add the garlic-chile mixture, onion and optional salt and cook, stirring occasionally, until the onion is translucent, about 3 minutes. Stir the mixture into the hot mashed plantains. (Alternatively, heat the lard and cook the aromatics in a large skillet, then add the hot mashed plantains; the texture will be more dense but just as good.)
Step 4Cook, stirring, until the flavors are well blended, 3 to 5 minutes. Serve at once. (You can make the dish several hours ahead, but it will seize up to a most forbidding texture when it cools and should be briefly reheated in a microwave to restore the original consistency.)