Step 1: Prepare the Beef

1. Slice the flank steak into thin strips, cutting against the grain for maximum tenderness. Pat the beef dry with paper towels.
2. Place the beef in a bowl and toss with the cornstarch until evenly coated. Shake off any excess cornstarch and set aside.

Step 2: Make the Sesame Sauce

1. In a medium bowl, whisk together the soy sauce, brown sugar, rice vinegar, hoisin sauce (if using), oyster sauce (if using), and water.
2. Set the sauce mixture aside. In a small bowl, combine 1 teaspoon of cornstarch with 1 tablespoon of water and mix until smooth.

Step 3: Cook the Beef

1. Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat.
2. Add the beef in a single layer, cooking in batches if necessary to avoid overcrowding. Sear for 1-2 minutes on each side until golden and crispy. Remove the beef from the skillet and set aside.

Step 4: Cook the Sesame Sauce

1. In the same skillet, add 2 tablespoons of sesame oil and reduce the heat to medium.
2. Add the minced garlic and ginger, sautéing for 30 seconds until fragrant.
3. Pour in the prepared sauce mixture and bring it to a gentle simmer. Cook for 2-3 minutes, stirring occasionally, until slightly reduced.

Step 5: Combine the Beef and Sauce

1. Return the cooked beef to the skillet, tossing it in the sauce until evenly coated.
2. Stir in the cornstarch slurry and cook for an additional 1-2 minutes until the sauce thickens to a glossy consistency.

Step 6: Garnish and Serve

1. Sprinkle the Sesame Beef with toasted sesame seeds and chopped green onions.
2. Serve hot over steamed rice, noodles, or alongside stir-fried vegetables. Enjoy!