Position the broiler pan 4 to 5 inches from the heating element. Preheat the broiler and broiler pan.
Combine in a small bowl combine the oli, garlic, pepper and salt. Rub the seasoning blend the tops of the mushrooms and place the mushrooms on the broiler rack. Use the last portion of the seasoning mixture to sesason the steak.
Place the steaks and mushrooms on the broiler pan. Cook for 8 to 12 minutes, turning the steaks and mushroom caps once halfway through cooking. Make a small incision and check the center. The interior will be slightly less done than desired, for it will continue to cook somewhat off the heat. Cook each side for 1 to 2 minutes more for medium. Remove the steaks and mushrooms from the broiler and let the steaks stand, loosely covered, for 5 minutes. Remove the mushrooms to a small bowl and toss with the balsamic vinegar.