Spray grate of outdoor grill and utensils with grilling spray. Preheat grill to medium heat according to manufacturer's directions.
Spray large skillet with grilling spray; heat over medium-high heat. Add mushrooms and onion; cook 8 minutes or until tender, stirring occasionally. Remove from heat. Stir in stir fry sauce; set aside.
Sprinkle steaks with seasoned salt and pepper. Grill 10 minutes or until medium done (160°F), turning over after 5 minutes.
Transfer steaks to serving plates. Spoon mushroom mixture evenly over steaks.