While the steak is frozen, trim the fat and slice paper-thin. Frozen meat is much easier to cut thin than thawed meat.
Place the steak strips in a large bowl and top with: Soy sauce, brown sugar, sesame oil, garlic, ginger, chopped onion whites, pepper, and sriracha. Toss to coat and allow the steak to thaw and marinate for 45-60 minutes.
Heat a wok (or large skillet) to high heat. Use tongs to transfer half of the meat to the hot wok. Spread the steak pieces around the skillet and allow them to sear on one side before using the tongs to move them around and caramelize on all sides. Once the juices have evaporated and the steak has a nice brown color, remove from the wok, 5-8 minutes.
Cover the cooked steak to keep warm. Repeat with the remaining meat, and throw out the leftover marinade. Serve warm, sprinkled with the reserved green onion tops and sesame seeds.
Keep leftovers Korean bbq in an airtight container in the fridge for up to 3 days, or in the freezer for up to 3 months.