In a bowl, combine olive oil, garlic powder, cumin, salt, and pepper. Rub the mixture all over the flank steak and let it marinate for at least 30 minutes (or up to 2 hours in the refrigerator for more flavor).
In a mixing bowl, combine cilantro, parsley, minced garlic, red wine vinegar (or lime juice), red pepper flakes, and olive oil. Stir well and season with salt and pepper to taste. Set aside for the flavors to meld.
Preheat a grill or skillet over medium-high heat. Sear the marinated flank steak for about 5-7 minutes on each side for medium-rare, or cook to your preferred doneness. Remove from heat and let it rest for 5 minutes.
While the steak is resting, warm the corn tortillas on the grill or in a skillet for about 30 seconds on each side until pliable.
Thinly slice the rested steak against the grain to ensure tenderness.
Take each tortilla and layer on slices of the grilled steak. Top with a generous spoonful of chimichurri and a couple of slices of avocado.
Squeeze fresh lime juice over the tacos before serving.
Notes
Let the steak marinate for longer for enhanced flavor.