Prepare the marinade. Use a citrus juicer to squeeze the lemon and lime juice. Mix the juices in one container, add the onion rings and sliced fish. Keep the fish in the marinade in a cool place for at least 20 minutes.
Prepare the vegetables. Place the tomatoes and sweet peppers on a serving dish, add salt, olive oil and pepper.
Finish cooking. Drain the fish and onions, add them to the vegetables, add a little sugar and mix. Let the ceviche sit in the refrigerator for another 10 minutes and serve.