Combine soy sauce, olive oil, lime juice, honey, toasted sesame oil, garlic, crushed red pepper flakes, and black pepper in a gallon-sized plastic bag. Place flank steak inside, turn it over in the marinade to coat, squeeze out the air, and seal the bag. Place the bag in a dish (in case of any leaks) and refrigerate for 6 to 12 hours, flipping the bag over halfway through if possible. Remove the meat from refrigerator at least 30 minutes before cooking so that it can start to come to room temperature.
To cook, grill over high heat for about 4 minutes per side, taking care not to overcook since meat is thin. (The center of the steak should register 130°F on an instant-read thermometer for medium-rare, or 135° for medium.) Remove steak from the grill and loosely tent it with foil for 10 minutes to allow the juices to redistribute. Thinly slice against the grain and serve immediately.