Dominican Mangu (Mashed Plantains)

Peel the plantains by trimming off both ends and making a shallow slit lengthwise through the skin. Remove the peel then cut each plantain in half and cut each half lengthwise, creating 4 pieces per plantain.
Place the plantains in a pot and add enough water to cover them by about 1 inch. Season the water with a pinch of salt. Bring to a boil and cook until the plantains are fork tender, about 25–30 minutes.
While the plantains cook, place the sliced red onion in a bowl and pour the vinegar over it. Let sit until ready to use.
Drain the cooked plantains, reserving ½ cup of the cooking water.
Transfer the plantains to a large bowl. Add the butter and ¼ cup of the reserved cooking water. Mash until mostly smooth. Add the ice or cold water and continue mashing until creamy. If needed, add more reserved cooking water a little at a time until you reach your desired consistency.
Add 2 tablespoons of the onion vinegar and the ½ teaspoon salt to the mangu. Mash and mix until well combined.
Heat the oil in a pan over medium heat. Add the onions and remaining vinegar, and cook for 3–5 minutes, or until the onions are softened and lightly pink.
Serve the mangu topped with the onions and with avocado slices. Buen provecho!