Peel the plantains (see how-to) and cut lengthwise, then divide each half into two. You can remove the center where the seeds are located (optional, this is just my preference for a smoother mangú).
Boil the plantains in enough water to cover them plus an inch until they are very tender, having added the salt to the water before the water breaks the boil.
Remove the plantains from the water and mash them right away with a fork until they are very smooth and there are few to no lumps (be careful not to burn yourself). Mix in butter, and progressively add a cup of water at room temperature and keep mashing and mixing until it turns into a very smooth puree (see tips below).
Onions
Heat a tablespoon of olive oil in a skillet over low heat. Add onions and cook and stir until they become translucent. Pour in vinegar and season with salt to taste.
Garnish mangu with the onions and serve per suggestions above.
Video
Notes
Keep in mind that once cooled, and by the time you get it to the table, it will become thicker so make it a bit thinner than you'd think you like it.