Peel plantains by cutting along the edges and then chop into 1-2 inch pieces. Add plantain to a pot and add enough water to cover 1 inch above the plantains.
Cook on medium heat until the plantain is soft and tender, about 30-35 minutes. Drain the plantain and save ½ of the cooking water.
In a large bowl, add the cooked plantains, butter, and reserved cooking water, then mash until there are no more chunks of plantain.
Add salt and cold water and continue mashing until smooth. Set aside as you prepare with the rest of the dish.
Sides
In a pan over medium heat, add enough frying oil to cover to bottom of the pan. Fry the eggs and cook to your liking. Season with salt, remove from pan, and set aside.
Add cheese slices to the pan and cook on both sides until golden brown. Remove from the pan and follow the same process for salami slices, cooking until golden brown, then remove from the pan.
In a small bowl, combine sliced onion, vinegar, and salt. Add onion slices and marinating vinegar to the same pan used for the sides and cook until translucent, about 3 minutes.
Assemble
Assemble dish with a scoop of mashed plantain, slices of fried salami, slices of cheese, fried egg, a quarter of avocado, and fried onions. Buen provecho!