Peel the plantains: Cut off the ends of the plantains, then make 3-4 shallow cuts all the way down each plantain, along the ridges. Gently separate the peel from the plantain flesh.
Cut the plantains in half lengthwise, then cut the halves in half lengthwise again. Remove the seeds if desired. Cut the 4 plantain pieces in half one more time but horizontally this time, so you have 8 short pieces. (How many pieces you have or how you cut them isn't crucial- you just want smaller pieces so they cook relatively quickly)
Boil 1- 1 1/2 liters of water in a medium pot over medium-high heat with a dash of sea salt.
Add the plantains to the water (make sure the water is enough to cover them completely). Boil for 20-25 minutes.
Drain the water (If not using chicken broth for your mangu, be sure to save 1 cup of the plantain water to add back while mashing). Start mashing the plantains with a fork or potato masher.
Add the chicken broth (or water), coconut oil, garlic, and 1 tsp salt. *Add the broth/water a little at a time up to 1 cup to desired thickness.
Onion Garnish
Heat the oil in a medium-sized pan over medium heat. Add the onions and cook for 5-10 minutes or until translucent.
Add the vinegar and salt. Cook for 3-4 more minutes, then remove the pan from the heat.
Serve the onions as a garnish on top of the mangu. Enjoy!
Notes
Storage: This dish will stay fresh in the fridge in an airtight container for up to about 3 days. To Reheat: Heat it on the stove or in the microwave. Mash as you reheat and add a little liquid as needed just to soften it.