• Thinly slice the red onion. Place the sliced onions in a bowl and add the vinegar and salt, to taste. Set aside.
• Fill a large pot with water and place it over high heat. Add 1/2 tbsp of salt.

Peel and cook green plantains

• Start by cutting off both ends of the green plantains and slicing a slit lengthwise down the center.
• Carefully separate the peel by pulling it away from the flesh.
• Cut each plantain into 4 pieces.
• Once the water comes to a boil, add the plantains and cook for about 15 minutes until fork tender.

Cook the onions

• Add oil to a small saucepan over low heat. Add the onions and vinegar while the oil is still cool to avoid splattering. Cook for a few minutes until the onions turn bright pink but still have some crunch to them. Set aside.

Mash the plantains

• Once cooked, remove the plantains from the boiling water and place them in a large bowl. Reserve about 1/2 cup of the boiling water. (You could add cold water instead.)
• While the plantains are still hot, add some butter, some of the pickled onion juice, and a splash of the reserved water and begin mashing the plantains. Repeat the steps until everything is nice and smooth.
• Taste the mangú and add salt, if necessary.
• Top with pickled onions and serve with fried eggs, fried salami and fried cheese. Enjoy! ¡Buen provecho!