Thinly slice the beef sirloin or flank steak against the grain, aiming for roughly ¼-inch thickness.

In a medium mixing bowl, combine soy sauce, brown sugar, grated ginger, minced garlic, rice vinegar, and sesame oil. Whisk until the sugar is dissolved.

Add the sliced beef to the marinade, ensuring every piece is coated, and let it marinate for at least 30 minutes at room temperature.

Heat a large skillet over medium-high heat and add oil. Once hot, add the marinated beef in a single layer and sauté for about 3-4 minutes until browned.

Pour in the remaining marinade and simmer for 2-3 minutes until slightly thickened.

Remove from heat, serve over rice or noodles, and garnish with green onions.