Preheat barbecue or broiler to hot.
Make parsley sauce in a bowl by whisking together the oil, vinegar, onion, parsley, capers and garlic and set aside.
Season the fish with garlic, lemon juice, salt and pepper, and brush or drizzle with oil.
Grill or broil fish for five minutes a side.
Transfer to heated platter, spread with parsley sauce, and serve.
