Combine marinade ingredients in small bowl. Place beef steaks and marinade in food-safe plastic bag; turn steaks to coat. Close bag securely and marinate in refrigerator 15 minutes to 2 hours.
Remove steaks from marinade; discard marinade. Place steaks on grid over medium, ash-covered coals. Grill, covered, 10 to 14 minutes (over medium heat on preheated gas grill, 12 to 16 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Meanwhile combine avocado salsa ingredients in medium bowl. Set aside.
Place tortillas on grid. Grill until warm and slightly charred. Remove; keep warm.
Carve steaks into slices. Serve in tortillas with avocado salsa. Top with onion, cilantro and lime wedges, as desired.
Nutrition information per serving, using flat iron steaks: 493 Calories; 171 Calories from fat; 19g Total Fat (5 g Saturated Fat; 9 g Monounsaturated Fat;) 68 mg Cholesterol; 407 mg Sodium; 45 g Total Carbohydrate; 7.4 g Dietary Fiber; 35 g Protein; 4.4 mg Iron; 6.1 mg NE Niacin; 0.7 mg Vitamin B6; 4.1 mcg Vitamin B12; 11.4 mg Zinc; 27.7 mcg Selenium.
This recipe is an excellent source of Dietary Fiber, Protein, Iron, Niacin, Vitamin B6, Vitamin B12, Zinc, and Selenium.