Arrange sliced red onion in large 16 oz. glass jar.
Bring water, vinegar, sugar, and salt to boil over medium high. Turn off as soon as the sugar and salt have dissolved, this should happen quickly around a minute.
Carefully, pour liquid mixture over the onion in the jar making sure to cover onion slices entirely.
Set aside to cool for at least 1 hour before using. These may be made ahead and stored for up to 2 weeks in the refrigerator.
Boil the plantains in water with 1 tsp. of salt until they are fork tender.
Drain, reserving at least 1 cup of cooking water to add back during mashing.
Mash them with a fork or potato masher until they are smooth, or slightly chunky if preferred. Add cooking water as needed tablespoons at a time, you may not need all the water to keep the plantains moist. Do not add water all at once.
Once mashed, add butter and any additional salt, mix to combine.
Set aside, covered, until all items are ready.
In a clean skillet, add oil and fry the slices of Queso de Freir over medium heat until golden brown on each side.
In the same skillet add oil and fry the slices of salami until golden brown on each side.
Fry the eggs to preferred doneness, place a large helping, approximately ½ cup, of mangú on each of 4 plates. Surround the plantains with 1 egg, 2 slices of Queso de Freir, and 2 slices of salami or salchichón per plate. Top plantains with pickled onions and serve.