Prepare the Sauce / Marinade:
In a bowl, whisk together soy sauce, mirin, sake, brown sugar, sesame oil, garlic, ginger, and chili flakes if using.
Marinate the Beef:
Place thinly sliced beef in a bowl or zip-top bag.
Pour about half of the sauce over the beef and marinate for 15–20 minutes. Reserve the rest of the sauce for cooking or drizzling.
Cook the Beef:
Heat a skillet or grill pan over medium-high heat with a touch of oil.
Cook the beef in batches (don’t overcrowd the pan), searing for 1–2 minutes per side, or until caramelized and cooked through.
Serve:
Drizzle with remaining sauce and sprinkle with sesame seeds and chopped green onions.
Serve over steamed rice, with shredded cabbage, kimchi, or wrapped in lettuce leaves.