Preheat the grill to high heat. Pat the grouper fillet dry then drizzle with olives oil. Sprinkle with minced garlic, salt and pepper. Then zest 1-2 oranges over the top of the grouper. Gently rub the grouper on both sides to distribute the orange zest, garlic and seasoning.
Slice the oranges and onions. Grill the grouper for about 4 minutes per side, carefully flipping with two spatulas, so the fillet doesn't break. Tent with foil, then grill the oranges and onions for 1 minute per side.
Place the grouper on a platter with the grilled onions, oranges and olives. Serve warm.