1 beef flank steak (about 1-1/2 to 2 pounds)
18 small corn tortillas (6 to 7-inch diameter)

Toppings:

Minced white onion, chopped fresh cilantro, lime wedges

Marinade:

1 cup prepared tomatillo salsa
1/3 cup chopped fresh cilantro
2 tablespoons fresh lime juice
2 teaspoons minced garlic
1/2 teaspoon salt
1/4 teaspoon pepper

Avocado Salsa:

1-1/2 cups prepared tomatillo salsa
1 large avocado, diced
2/3 cup chopped fresh cilantro
1/2 cup minced white onion
1 tablespoon fresh lime juice
1 teaspoon minced garlic
1/2 teaspoon salt

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 6 hours or overnight. Remove steaks from marinade; discard marinade. Place steaks on grid over medium, ash-covered coals. Grill, covered, 17 to 21 minutes for medium rare to medium doneness, turning once. 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.