Cut beef steak lengthwise in half, then crosswise into 1/4-inch thick strips. Combine beef and taco seasoning mix in medium bowl; stir to coat beef.
Heat 1 teaspoon oil in large, nonstick skillet over medium-high heat until hot. Add half of beef; stir-fry 1 to 2 minutes or until outside of beef is no longer pink. Remove from skillet. Repeat with 1 teaspoon oil and remaining beef. Set aside; keep warm.
Heat remaining 1 teaspoon oil in same skillet until hot. Add onion; cook 2 to 3 minutes or until tender. Return beef to skillet; cook 1 minute or until heated through.
Place equal amounts of steak and onion on each tortilla; top with lettuce, cheese, salsa and mango.
Nutrition information per serving: 574 Calories; 144 kcal Calories from fat; 16 g Total Fat; 4 g Saturated Fat; 4 g Monounsaturated Fat; 75 mg Cholesterol; 1647 mg Sodium; 62 g Total Carbohydrate; 8.4 g Dietary Fiber; 44 g Protein; 2.2 mg Iron; 8.2 mg Niacin; 0.7 mg Vitamin B6; 1.4 mcg Vitamin B12; 5.1 mg Zinc; 31.3 mcg Selenium; 110.9 mg Choline;