Preparation

For the pickled onions:
• Place the sliced onions in a glass jar or bowl.
• In a saucepan, combine the vinegar, water, sugar, salt, garlic and allspice over medium-high heat, stirring occasionally until the salt and sugar dissolve. Remove from heat and let cool slightly.
• Pour the brine over the onions. Let cool to room temperature, then cover and refrigerate overnight.

For the tostones:
• Trim the ends off each plantain, cut through the tough outer peels lengthwise top-to-bottom and remove. Slice into 1-inch rounds.
• Add the oil to a large, heavy-bottomed skillet (it should fill about ¹∕³ the skillet), and place over medium heat. When the oil is hot, add the plantains and fry until softened and golden brown, about 4 minutes per side.
• Place the fried plantains on a cutting board or plate and gently smash and flatten each piece with the bottom of a glass or plate.
• Increase the heat to medium-high and return the smashed plantains to the oil, frying until crispy, about 1 minute per side. Place on a paper towel-lined plate or cooling rack and sprinkle with sea salt.
• Prepare the avocados by cutting them in half, removing the pits, scoring the flesh into cubes and scooping into a small bowl. Add the lime juice and salt and slightly smash with a fork until the avocado mixture is chunky but combined.
• Plate the plantains, top each with 1 to 2 tablespoons avocado mixture and 2 to 3 slices of red onion. Garnish with cilantro and lime wedges.