In a large bowl, combine beef, soy sauce, garlic, and peppercorns. Let marinate for at least 30 minutes or overnight for deeper flavor.
Heat oil in a deep pan over medium heat. Sauté onions until soft and fragrant. Add marinated beef (reserve the marinade) and sear until browned on all sides.
Pour in the reserved marinade, vinegar, and water. Add bay leaves. Bring to a boil without stirring, then lower heat and simmer covered for 60–90 minutes or until beef is fork-tender.
Add brown sugar (if using) and simmer uncovered for another 10 minutes to reduce and thicken the sauce.
Serve hot over steamed rice, garnished with pickled vegetables or fresh tomato slices if desired.