Cut the ends off the green plantains and score the skin lengthwise. Use your thumb or a knife to peel them, then cut the plantains into 1-inch thick rounds.
Place the chunks in a large pot of cold water with a tablespoon of salt. Bring to a boil and cook for 20 to 30 minutes until they are tender enough for a fork to pass through easily.
While the plantains boil, slice a red onion into thin rings. Soak them in white vinegar and a pinch of salt for 10 minutes, then sauté them in a pan with a little oil until they are soft and bright pink.
Once the plantains are cooked, drain the water but keep at least one cup of the hot starchy cooking liquid in a separate bowl.
Add butter to the hot plantains and begin mashing while they are still steaming. Do not wait for them to cool, or they will become lumpy and hard.
Gradually add the reserved hot water (or a splash of ice-cold water) as you mash. Continue mashing until the consistency is velvety, light, and free of any solid chunks.
Plate and Garnish: Serve the mash immediately while hot. Top the Mangú with the sautéed onions and a spoonful of the vinegar from the pan to cut through the richness.
