01 - In a large mixing bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, rice vinegar, gochugaru (if using), and black pepper until the sugar is dissolved and the mixture is well combined.
02 - Add the thinly sliced beef to the marinade and toss thoroughly to ensure every piece is evenly coated. Allow the beef to marinate for at least 10 minutes at room temperature, or up to 1 hour in the refrigerator for deeper flavor penetration.
03 - Heat a large skillet or wok over high heat until it begins to smoke. Add the marinated beef in a single layer and cook for 3 to 4 minutes, stirring occasionally, until the beef is deeply browned and cooked through. Avoid overcrowding the pan to achieve a proper sear.
04 - Remove the skillet from heat and stir in half of the sliced green onions and 1 teaspoon of sesame seeds, reserving the remainder for garnish. Toss gently to distribute the aromatics throughout the beef.
05 - Spoon the hot beef and sauce over bowls of steamed white rice. Garnish each serving with the remaining green onions and sesame seeds. Serve immediately.