To prepare steak, sprinkle salt and pepper evenly over both sides of steak. Place steak on a broiler pan coated with cooking spray; broil 5 minutes on each side or until desired degree of doneness. Remove the steak from oven; loosely cover with foil.
To prepare sauce, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add 1/2 cup onion and garlic; sauté 2 minutes. Add mushrooms; sauté for 4 minutes. Add broth, wine, and vinegar. Bring to a boil; reduce heat to medium, and cook until reduced to 1 1/4 cups (about 6 minutes). Add green onions, and cook for 1 minute.
Slice steak diagonally across the grain into 1/4-inch slices. Serve steak with mushroom sauce.