Heat a skillet to medium-high heat. Spray well with cooking spray and add mushrooms. Sprinkle with salt and pepper and cook for about 10 minutes or until soft.
Stir in onion and thyme and cook for about 2 minutes. Add beef broth and reduce heat to simmer for 5 minutes.
Using a grill or grill pan, cook the steak for about 12 minutes- turning once. Or until desired doneness. Remove from heat, let rest for a few minutes. Thinly slice the meat against the grain.
Spread sliced steak on a plate, top with mushroom sauce and serve.