In a small skillet, heat the 1 tablespoon of olive oil. Add the onion and garlic, cover and cook over moderate heat, stirring occasionally, until very soft and browned, about 7 minutes. Peel the baked plantain and add it to the skillet. Using a fork, mash the plantain until smooth and incorporated with the onion and garlic. Season with salt and pepper. Cover the mashed plantain mixture and keep warm.