Prep the plantains: Wash the plantains and peel off the skin. Cut them into thin rounds or batons, about 1/4 inch thick.
Boil the plantains: Bring a large pot of salted water to a boil. Add the plantain slices and cook for 5-7 minutes, or until just tender but not mushy. Drain the plantains well and set them aside to cool slightly.
Make the marinade: In a large saucepan or Dutch oven, heat the olive oil over medium heat. Add the sliced onion and bell peppers (if using) and cook until softened and translucent, about 5 minutes.
Add the vinegar, garlic, bay leaves, oregano, salt, and peppercorns to the pan. Bring to a simmer and cook for 2-3 minutes, allowing the flavors to meld.
Assemble and marinate: Add the cooled plantains and olives to the marinade. Gently stir to coat the plantains evenly. Remove the pan from heat and let it cool completely to room temperature.
Chill and serve: Transfer the Escabeche de Guineos to an airtight container and refrigerate for at least 2 hours, or preferably overnight, to allow the flavors to develop fully. Serve cold as a side dish.