Preheat grill to high heat. Pat the fish fillet and drizzle with olive oil. Sprinkle with garlic, salt and pepper. Add the orange zest and gently rub the fish on both sides to distribute seasonings and place on oiled baking tray.
Grill the fish for about 3 minutes per side, carefully flipping the fillet so that it doesn't break. Add the oranges and onions and grill for about 2 minutes each side.