Thinly slice your beef sirloin or flank steak against the grain to ensure tenderness.

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

Add the sliced beef to the marinade, ensuring each piece is coated. Cover and let sit at room temperature for about 30 minutes.

Heat a large skillet over medium-high heat with a tablespoon of oil. Add the marinated beef, cooking for 3-4 minutes until browned.

After removing beef, pour any leftover marinade into the skillet and simmer for 3-5 minutes until the sauce thickens.

Plate the beef teriyaki over rice or noodles, drizzling the thickened sauce on top and garnishing with sliced green onions.