Preheat the oven to 350 degrees.  Heat the oil in a large skillet over medium heat.  Add the onion and garlic and cook until soft.  Add the tomatoes and saute until the tomatoes break down completely.  Salt and pepper to taste.  Arrange the sea bass in a baking dish.  Pour the sauce over the fish.  Sprinkle the fish with the olives and capers.  Bake about 15 minutes.