PREHEAT oven to 350 degrees.
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.
WHEN ready to bake fish, transfer to a 9x13-inch baking dish along with the olive oil, garlic, and onion.
SPRINKLE the fish with the cayenne or white pepper.
BAKE for 30 minutes.
IF GRILLING: Wrap the fish and oil, garlic, onion, and pepper in foil.