Place sliced beef in a large bowl.
In a separate bowl, whisk together soy sauce, brown sugar, garlic, sesame oil, onion, and ginger.
Pour marinade over beef and toss to coat evenly. Cover and refrigerate for at least 30 minutes (up to overnight).
Heat a large skillet over high heat. Add a small amount of oil if needed.
Cook beef in batches, spreading it into a single layer. Cook for 2–3 minutes per side until caramelized and cooked through.
Remove from heat and garnish with green onions and sesame seeds. Serve immediately.