To make mangú: 

Peel the plantains and cut lengthwise, then divide each half into two. 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 with a fork until they are very smooth and there are few to no lumps. Mix in olive oil, onion powder and water at a cool temperature and keep mashing and mixing until it turns into a smooth puree.

To make onion garnish: 

Heat a tablespoon of 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 with sunny side-up eggs or Dominican scrambled eggs, Dominican fried cheese or fried slices of salami.