Sprinkle steak with salt and pepper. Grill steak, covered, over medium heat for 8-10 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°).
Meanwhile, combine the spinach, red pepper, tomatoes and onion; divide among four serving plates. In a small bowl, combine the salad dressing, picante sauce and taco seasoning. Slice steak against the grain; arrange over salads. Drizzle with dressing.