Cut off both ends of plantains. Peel by passing knife through skin and carefully lifting it off with knife. Once peeled, cut each plantain in half lengthwise.

In large pot, cover plantains with 3-4 cups water; add 1 tsp. salt. (Note: Water must fully cover plantains.) Cook 30-35 minutes until plantains are soft when poked with fork.

In small pot over medium heat, cook onions, vinegar, avocado oil and 1 tsp. salt until translucent but still crunchy, 5-6 minutes. Remove from heat and set aside. Do not drain.

Drain water from plantains, leaving plantains in pot. Using masher or fork, mash plantains until smooth; mix in butter. (Note: For creamier mash, add ¼ cup of cold water.)

Pour warm liquid from onions into plantains and fully combine.

To serve, top mangú with onions.