Rub the fish fillets with the crushed garlic, then place them in a shallow, non-reactive dish. Spoon the olive oil over the fish until they are coated. Place the onion on top of the fish. Cover the fish and refrigerate overnight to allow the fish to soak in the marinade.
Preheat the oven to 180 degrees C.
If baking the fish, transfer it to a 20x30cm baking dish along with the olive oil, garlic and onion. Sprinkle the fish with the cayenne or white pepper.