Step 1: Prepare the Beef

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

Step 2: Make the Sauce

1. In a medium bowl, whisk together the soy sauce, water, brown sugar, hoisin sauce (if using), rice vinegar (if using), and red pepper flakes. Set the sauce aside.

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 the beef for 1-2 minutes on each side until golden brown and crispy. Remove the cooked beef from the pan and set aside.

Step 4: Make the Sauce and Combine

1. In the same skillet, heat 2 tablespoons of oil over medium heat. Add the minced garlic and ginger, sautéing for 30 seconds until fragrant.
2. Pour in the prepared sauce mixture, stirring well to combine. Bring the sauce to a simmer and cook for 2-3 minutes until slightly thickened.
3. Return the cooked beef to the skillet, tossing it in the sauce until fully coated. Cook for an additional 1-2 minutes to ensure the beef is heated through and the sauce clings to the meat.

Step 5: Garnish and Serve

1. Remove the skillet from heat and garnish the Mongolian Beef with chopped green onions and sesame seeds.
2. Serve hot over steamed white rice or noodles, and enjoy!