Boil the plantains. Start by bringing a pot of salted water to a boil. Peel and slice the plantains into slices small enough to fit into your pot. Once the water is boiling, toss in the plantains and boil until fork tender, about 20 minutes.
Mash the plantains. Strain the plantains from the water and transfer them to a pilon or bowl with the butter. Mash until they become consistent and creamy, adding water a few tablespoons at a time, until the texture is where you want it. Season with salt to taste.
Serve. Enjoy the mangu with the fried pickled onions on top. Feel free to serve with a fried egg, fried white cheese, and Dominican sausage.