1In a large bowl, combine the soy sauce, brown sugar, grated garlic, sesame seed oil, and black pepper. Mix well until the sugar is dissolved.
Add the thinly sliced beef short ribs to the marinade and toss to coat evenly. Cover and let marinate for at least 30 minutes (or up to 2 hours for more flavor).
Heat a large skillet or wok over medium-high heat. Add the marinated beef (including the marinade) and cook for 2-3 minutes, stirring frequently.
Add the sliced onion and continue to stir-fry for another 3-4 minutes, until the beef is cooked through and the onions are tender.
Pour in the water and stir well, allowing the sauce to simmer and thicken slightly for 2 minutes.
Taste and adjust seasoning if needed. Serve hot with steamed rice or your favorite side dishes.