In a medium saucepan add the bananas, water, salt to taste and cook for 20 minutes or until they are soft. Drain, mash until it forms a puree, add the butter and salt to taste.

Prepare the onion. In a small frying pan, add the olive oil, heat and cook the onion and sauté for several minutes and add the vinegar.

Serve the onions over the mangú.