Preheat your oven to 425°F (220°C).
In a large bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and rice vinegar.
Add the thinly sliced beef to the marinade, ensuring each piece is well coated. Let it marinate for at least 20 minutes, or up to 2 hours in the fridge.
On a large sheet pan, arrange the marinated beef in a single layer. Add the broccoli, bell pepper, and onion on top and drizzle with any remaining marinade.
Roast in the preheated oven for 15 to 20 minutes, or until the beef is cooked through and the vegetables are tender.
Remove from the oven and let it rest for a few minutes before serving. Garnish with sesame seeds.