In a medium bowl, combine the soy sauce, brown sugar, sesame oil, garlic, ginger, and rice vinegar. Whisk everything together until the sugar is fully dissolved and the marinade is smooth. This is where the magic happens, so take a moment to appreciate the aroma of the ginger and garlic blending together.

Slice the beef against the grain into thin strips for maximum tenderness. Add the beef slices to the marinade, making sure each piece is coated. Cover the bowl and refrigerate for at least 30 minutes or up to 2 hours for even more flavor. The longer it marinates, the more deliciously tender and flavorful your beef will be.

Heat a large skillet or wok over medium-high heat. Once hot, add the marinated beef in batches, making sure not to overcrowd the pan. Sear the beef on each side for about 2-3 minutes until browned and cooked through. Remove the beef from the skillet and set aside.

In the same skillet, pour in the remaining marinade and bring it to a simmer. Mix the cornstarch with the water in a small bowl to make a slurry, then slowly add it to the simmering sauce. Stir constantly until the sauce thickens to your desired consistency.

Return the cooked beef to the skillet and toss it in the thickened sauce, ensuring each piece is well-coated. Allow it to simmer for an additional 2-3 minutes to let the flavors meld together.

Serve your teriyaki beef hot, garnished with sesame seeds and green onions. Pair it with a side of steamed rice or noodles for a complete meal.