First, start by slicing the red onion and adding them to a bowl with white vinegar. Let them soak as you prepare the rest of this mangu recipe.
Peel and chop the green plantains. Add enough water to cover the plantains and salt to a large pot and bring everything to a boil. Boil the plantains until they are tender. This should take about 15-20 minutes.
Reserve 1/2 cup of water from the pot you boiled the plantains in. Drain the rest of the water. Add butter to the plantains and begin mashing them with a fork or masher. Add in the reserved cup of water and an additional 1/2 cup of cold water and continue mashing until you achieve a smooth consistency. The cold water is to keep the plantains from hardening. This may seem like a lot of water but the plantains will absorb it. You may add more salt to taste.
Add canola or vegetable oil to a pan over medium high heat. The ideal temperature for the oil is between 360-365 degrees F.
Fry the sliced salami for about 2-3 minutes on each side.
You can use the same pan with the same oil if you want the cheese to have a little flavor. Add the cheese and fry for 30 seconds to 1 minute on each side or until golden brown.
Fry the egg either in the same pan with the same oil or a different pan with new oil if desired. Cook until the edges are crispy and golden, about 3-4 minutes.
In a separate pan over medium heat, add oil. Cook the pickled onions along with the vinegar until they soften for about 2-3 minutes.
Serve the mangu topped with the pickled red onion slices, fried salami, cheese and egg.