Place the first seven ingredients in small skillet, over medium heat, to make the marinade. Heat until mixture is hot (2 to 3 minutes); let cool.
Place flank steak in a large resealable plastic bag; add cooled marinade and cilantro leaves. Refrigerate for at least 4 hours.
One hour before grilling, take meat out of refrigerator and let sit on counter.
Heat grill to medium-high heat.
Remove steak from marinade and discard leftover marinade.
Cook steak for 5 minutes on first side, turn over and cook an additional 3 to 5 minutes or until thermometer reaches at least 145° F.
Slice steak across grain into thin slices.
Place several slices of steak in the center of one corn tortilla, add ¼ cup of shredded lettuce, 1 tbsp. shredded queso cheese, 1 tbsp. of salsa, and 1 slice of avocado. If desired, garnish with a dollop of sour cream and a slice of lime. Repeat with remaining tortillas and ingredients.
