Sprinkle the steak with 1/2 teaspoon of the salt and the pepper. Spray a nonstick grill pan with cooking spray and set over medium-high heat. Add the steak and cook 5 minutes on each side for medium-rare. Transfer the steak to a cutting board and let rest 5 minutes. Cut the steak on an angle against the grain into 12 slices.
Meanwhile, whisk together the oil, vinegar, mustard and remaining 1/4 teaspoon salt in a large bowl. Add the lettuce, tomatoes and onions and toss to coat well. Transfer the salad to a platter. Top with the steak slices and sprinkle with the blue cheese and bacon. Serve immediately.