Cut the fish into 4 equal-size portions. Combine the olive oil, lemon juice, salt, pepper, garlic, and rosemary in a bowl. Brush the mixture onto the fish. To grill, place on grill rack and cook until the fish flakes easily, turning once. To broil, spray broiler pan with olive oil and arrange fish. Broil 4" from the heat for 4-6 minutes per 1/2" of thickness. If the fish is more than 1" thick, gently turn it halfway through broiling. To serve, top the fish with capers. Garnish with rosemary sprigs and serve.