Preheat broiler. Season steak with salt and pepper to taste. Place on a broiler rack; broil about 4 inches from heat until desired doneness is reached, about 4 to 5 minutes on each side for medium-rare.

Meanwhile, in a large nonstick skillet heat oil over medium-high heat. Add mushrooms and onion; sauté until lightly browned and liquid evaporates, about 6 minutes. Stir in salsa, cook, mixing gently, until salsa is hot, about 15 seconds. Carve steak diagonally across the grain in thin slices and serve with Fresh Mushroom Salsa Sauce.