In a bowl, combine the onions, oil, orange and lime juices, cilantro, garlic, salt, cumin, and cayenne. Place the fish in a large, non-reactive baking dish. Pour the marinade over the fish, cover and refrigerate for up to 1 hour.
Preheat a grill, and lightly oil the grill rack with vegetable oil.
Remove the fish from the marinade. Place on the oiled grill and cook, skin side up, until marked, 45 seconds to 1 minute. Turn carefully with a spatula and cook, skin side down, until the fish is just cooked through, about 4 minutes, depending upon thickness. Remove from the grill.
Spoon the coconut rice into the center of 4 large plates. Arrange 1 fillet onto each serving of rice and top with the Tropical Salsa. Garnish with toasted coconut and cilantro, and serve.
Yield: 4 servings