Remove skin and bones of fish and cut fish into small pieces. Wash and dry fish and place in a nonmetallic container. Sprinkle fish with salt and pepper, cover with onions, chiles, lime and lemon juice and mix well. Cover and marinate 3 to 4 hours in the refrigerator, stirring every hour.

Meanwhile, bring a pot of salted water to a boil, add corn and cook for 4 minutes. Drain the corn and slice each ear into 4 pieces. Wash and peel the sweet potatoes and cut into quarters. Set in a saucepan with lightly salted water to cover. Bring to a boil, cover loosely and boil 10 to 15 minutes or until potatoes are tender when pierced with a knife. Drain, cool and slice potatoes.

To serve, line a large platter with lettuce leaves. Place fish and onions in the center and surround with quartered ears of corn and sweet potato slices. Drizzle with extra virgin olive oil.