Prepare the Marinade: In a bowl, mix the soy sauce, sesame oil, honey, minced garlic, grated ginger, rice vinegar, and black pepper. Stir well until combined.
Marinate the Boneless Flanken: Place the boneless flanken strips in a resealable plastic bag or shallow dish and pour the marinade over the meat. Seal or cover and refrigerate for at least 1 hour, or preferably overnight for maximum flavor.
Preheat the Grill: Preheat the grill to medium-high heat (about 375°F to 400°F). If using a grill pan, heat it over medium-high heat on the stovetop.
Grill the Meat: Once the grill is hot, place the marinated boneless flanken strips on the grill. Cook for 3–4 minutes per side, flipping once, until they are browned and slightly charred on the outside, with a nice sear.
Rest the Meat: After grilling, let the meat rest for 5 minutes to allow the juices to redistribute.
Serve: Slice the grilled boneless flanken against the grain and garnish with sesame seeds and fresh cilantro if desired. Serve with your choice of sides like rice, grilled veggies, or in tacos.