Trim the ends of the plantains, score the peels lengthwise, peel, and cut into 1-inch chunks.
Bring 3 quarts water to a boil in a medium pot and add 2 tsp of the kosher salt. Add the plantains and boil until very tender and edges look slightly ragged, 18–22 minutes.
While the plantains cook, warm the vegetable oil in a skillet over medium heat. Add the red onion and 0.25 tsp of the kosher salt; cook, stirring, until softened but still slightly crisp, 3–5 minutes. Stir in the white vinegar and cook until the onions turn bright pink and the sharpness mellows, 30–60 seconds. Remove from heat.
Drain the plantains very well and return them to the pot. Add the unsalted butter and mash, drizzling in the cold water a little at a time, until smooth, thick, and fluffy with no dry lumps, 2–3 minutes. Season with the remaining 0.25 tsp kosher salt.
Mound the mangĂş on a platter or plates and spoon the onions and their juices over the top. Serve hot, optionally alongside fried eggs, Dominican salami, and queso frito.