Prepare the Beef: Start by placing the flank steak or sirloin in the freezer for about 15 minutes. This helps firm up the meat, making it easier to slice. After it firms up, carefully remove it and slice it thinly against the grain.

Coat the Beef: In a mixing bowl, toss the sliced beef with 1 tablespoon of cornstarch, ensuring each piece is well-coated. This helps achieve a slightly crispy texture when searing.

Whisk the Sauce: In a separate bowl, combine soy sauce, brown sugar, mirin, water, sesame oil, grated ginger, and minced garlic. Whisk until the brown sugar is dissolved and the mixture is smooth.

Make the Slurry: Mix cornstarch with water to make a slurry until the cornstarch is completely dissolved.

Sear the Beef: Heat oil in a skillet over high heat and sear the beef in batches for about 1-2 minutes per side until browned and cooked through.

Simmer the Sauce: Pour the teriyaki sauce into the pan over medium heat and allow it to simmer for 2-3 minutes, stirring gently.

Thicken the Sauce: Add the slurry to the sauce and stir continuously until thickened to your desired consistency.

Combine Beef and Sauce: Return the seared beef to the pan and toss gently in the thickened sauce to coat each piece evenly.

Garnish and Serve: Garnish your Beef Teriyaki with sesame seeds and chopped green onions. Serve hot over rice or alongside your favorite side dishes.