Cut ends of off the plantains and peel. Cut them into chunks. You may find it easier to cut them first and then peel.
Bring the plantains to a boil, then lower heat, cover and simmer until tender. This will typically take 20 to 30 minutes. Test with fork for tenderness. Keep the bananas in broth until you are ready to mash them.
In a separate pan cook onions in olive oil for 3 to 5 minutes on low heat. Add garlic and sauté for a few more minutes. Mash the bananas with the broth (add 1/2 of a cup of broth or water at a time until desired consistency – should be like mashed potatoes.) Add in oil, onions and garlic. Add in lemon juice and season with salt and pepper to taste. I personally also like to add some chopped fresh parsley or cilantro.
Serve as a side dish or you can top with bacon bits, pork cracklings (Chicharrón) or masas de puerco (fried pork chunks) and have as a meal.