Wash plantains, using a knife cut off the ends and discard, make a straight cut along the skin and the length of the plantains, making sure you are only cutting the skin and not deep into the flesh. Remove the skin with your fingers and discard. Roughly chop plantains into large chunks.
Bring water to boil in a large pot over medium-high heat, add the plantain and reduce to simmer for 20-25 minutes or until plantain is tender.
Remove pot from the heat, drain water except for 1/4 cup saved for mashing the plantain chunks.
Using a fork or potato masher,] add reserved liquid, vegan butter, and salt to taste. mash plantains until it resembles mash potato.
Heat oil in a skillet over medium-high heat, add onion slices and cook until soft.
Add lime juice and stir. Pour over the mangu and serve immediately.