Preheat grill or broiler. Sprinkle steaks with salt and pepper. Cook over medium heat for 8 to 12 minutes or until desired doneness, turning once or twice.
Meanwhile, heat 1 tablespoon olive oil in a large nonstick skillet over medium heat. Add mushrooms and onions, and stir-fry 10 minutes or until onions are soft and golden brown.
Cut each steak in half to make 4 servings and top with the mushroom mixture.