In a medium-size bowl, combine fish and salt and pepper and allow to set for 5 minutes. Add garlic and lime juice and cover the fish completely with liquid. Cover with plastic wrap and refrigerate for 40 minutes.
In another medium-size bowl, soak the onion in cold water for 20 minutes, then drain.
Drain all but a small amount of the lime juice from the fish, then combine with the remaining ingredients. Serve immediately chilled.