Begin by thinly slicing the beef (sirloin or flank steak) against the grain.

In a large mixing bowl, combine soy sauce, brown sugar, grated ginger, minced garlic, rice vinegar, and sesame oil. Whisk until the sugar fully dissolves.

Add the sliced beef to the bowl with the marinade and let it marinate at room temperature for at least 30 minutes.

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

Pour any remaining marinade into the skillet and let it simmer for 2-3 minutes until the sauce thickens.

Remove from heat, transfer the beef to serving plates, and garnish with chopped green onions.