Marinate the beef in soy sauce, brown sugar, sesame oil, garlic, ginger, and black pepper for at least 30 minutes.
Heat a large skillet or wok over medium-high heat.
Add the marinated beef to the skillet and cook until browned, about 5-7 minutes.
Remove the beef from the skillet and set it aside.
In the same skillet, add sliced onions and bell peppers.
Sauté the vegetables until they are tender, about 3-4 minutes.
Return the cooked beef to the skillet with the vegetables.
Stir to combine and heat through for another 2 minutes.
Garnish with chopped green onions and sesame seeds.
Serve hot over steamed rice or noodles.