In a small bowl, whisk together the olive oil, balsamic vinegar, soy sauce, minced garlic, minced cilantro, minced oregano, black pepper, chili powder, and chili flakes (if using).
Place the flank steak in a large bowl, shallow pan, or resealable plastic bag.
Pour the marinade over the steak and coat well so every inch is covered with marinade.
Marinate for at least an hour or refrigerate until ready to cook. If it will be more than an hour, refrigerate and marinate for up to 24 hours.
When ready to grill, drain the flank steak and remove any excess marinade. Discard the marinade. Set aside.
Prepare the smoker according to the manufacturer's instructions, and preheat it to 225-250°F/107-120℃.
Place the flank steak directly on the smoker grate and smoke for 75-90 minutes, or until the steak's internal temperature reaches 110-115°F/43-46℃. Remove steak from the smoker and place aside .increase the temperature to medium-high (450°F-500°F/230-260℃).
Sear the flank steak for 2-3 minutes on each side (or until it develops good grill marks, sears on the outside, and it reaches the desired doneness). If cooking for a crowd, the best internal temperature is 135°F/57℃.
Take the steak out of the smoker and cover it with foil to keep it warm.
Let it rest for 10-15 minutes for the juices to reabsorb into the steak. Slice against the grain and serve with favorite sides or use for tacos.