In a small bowl or shallow dish, whisk the olive oil, balsamic vinegar, soy sauce, minced garlic, cilantro, oregano, black pepper, chili powder, chili flakes (if using), and salt.
Pour the marinade over the steak, rub or press the steak into the marinade, making sure to evenly coat it with the marinade. Cover with plastic wrap, and let it marinate in the fridge for an hour or overnight.
Or put the marinade ingredients all in a resealable plastic bag, put the flank steak in the bag with the marinade, and chill it until ready to smoke or grill.
When ready to cook, drain the steak, pat it dry, and put it on a preheated grill or smoker.
Notes
An instant-read meat thermometer will tell you when the steak reaches the desired doneness.
Avoid marinating flank steak for more than 24 hours to prevent it from turning into mush.
Add chili flakes or cayenne pepper for a spicier kick.
Feel free to adjust the seasonings to your tastes. A touch of honey or brown sugar will make it slightly sweeter.
Please remember that the nutritional information is a rough estimate for the marinade only, and can vary greatly based on the ingredients used in the recipe.