In a medium bowl, combine soy sauce, rice wine vinegar, brown sugar, sesame oil, minced ginger, and minced garlic. Whisk until well blended and set aside.

Heat a skillet over medium heat, add ground beef and diced onion. Cook for about 5 minutes until browned and onion is translucent.

Pour the bulgogi sauce over the beef and simmer for 6-10 minutes until the sauce thickens and is fragrant.

Remove from heat and serve in bowls, garnished with scallions and sesame seeds, and optionally a soft-boiled egg.