Combine Lime Vinaigrette ingredients in small bowl. Place beef steak and 1/4 cup marinade in food-safe plastic bag; turn steak to coat. Close bag securely and marinate in the refrigerator 15 minutes to 2 hours. Cover and refrigerate remaining vinaigrette for salad.
Remove steak from marinade; discard marinade. Place steak on grid over medium, ash-covered coals. Grill, covered, 11 to 15 minutes (over medium heat on preheated gas grill 13 to 16 minutes) for medium rare (145°F) to medium doneness (160°F), turning occasionally.
Divide salad greens among 4 plates. Top evenly with mango pieces, avocado slices and onion, as desired. Carve steak into slices. Divide steak slices evenly over salads. Sprinkle with cheese and pepitas, if desired. Drizzle remaining vinaigrette evenly over salads. Season with salt and pepper, as desired.
Nutrition information per serving: 352 Calories; 153 kcal Calories from fat; 17 g Total Fat; 4 g Saturated Fat; 6 g Monounsaturated Fat; 72 mg Cholesterol; 186 mg Sodium; 24 g Total Carbohydrate; 3.4 g Dietary Fiber; 30 g Protein; 2.7 mg Iron; 11.1 mg NE Niacin; 0.7 mg Vitamin B6; 1.5 mcg Vitamin B12; 5.1 mg Zinc; 31 mcg Selenium; 106.1 mg Choline;