Sprinlke salt and pepper evenly over both sides of steak. Place steak on a broil pan coated with cooking spray; broil 5 minutes on each side or until desired degree of donesness. Remove the steak from the oven; loosely cover with foil.
Heat a large non-stick skillet over medium-high heat. Coat pan with cooking spray. Add 1/2 cup onion and garlic; saute 2 minutes. Add mushrooms; saute 4 minutes. Add broth, wine, and vinegar. Bring to a boil; reduce heat to medium and reduce 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.