Press pepper into both sides of steak. Place steak on a broiler pan coated with cooking spray. Broil 8 minutes on each side or until steak is done.
Remove and discard stems from mushrooms. Heat oil in a large skillet over medium heat. Add mushroom caps and green onions, and cook 3 minutes, stirring frequently. Add pimento, garlic, vinegar, and salt. Cook 1 minute.
Cut steak diagonally across grain into thin slices. Spoon mushroom mixture over steak.