Beef Flat Iron Steaks get big south-of-the-border flavor from cilantro, onions and tomatillos.
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.