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.
c.1997, M.S. Milliken & S. Feniger, all rights reserved