1. Heat oil in large skillet until nearly smoking. Season fish with salt and pepper. Place fish fillet, flesh side down, in hot skillet. Add onions to pan and cook, turning them after one minute. (Caramelizing them is the key to the dish.) Turn fish after 2 minutes. Add capers, tomatoes, white wine. Cover pan and let cook for 3 minutes. Check doneness of fish. When cooked almost completely through, transfer fish to serving platter.
2. Add butter to skillet and stir quickly to thicken sauce, taste and adjust seasoning, if needed. Pour sauce over fish fillet and serve.