1. Prepare the Flank Steak: Heat 2 tbsp of vegetable oil in a large skillet or wok over medium-high heat. While the oil heats up, thinly slice the flank steak against the grain. Once the oil is hot, add the steak slices to the skillet in a single layer. Cook the steak for about 5-7 minutes, stirring occasionally until it is browned and cooked through. Remove the steak from the skillet and set it aside.

2. Make the Sauce: In the same skillet, lower the heat to medium and add 1/2 cup soy sauce, 1/2 cup brown sugar, 1/4 cup water, 3 minced garlic cloves, and 1 tbsp fresh minced ginger. Stir the ingredients together and bring the mixture to a gentle simmer, allowing the sugar to dissolve and the flavors to blend.

3. Thicken the Sauce: In a small bowl, mix 1 tbsp cornstarch with a few tablespoons of water to form a slurry. Slowly pour the slurry into the simmering sauce while stirring continuously. Cook for about 2 minutes or until the sauce has thickened to your desired consistency.

4. Combine the Steak: Return the browned flank steak to the skillet with the thickened sauce. Toss the steak in the sauce to ensure it’s fully coated. Let it cook for an additional 2 minutes to allow the steak to soak up the flavors and heat through.

5. Garnish and Serve: Once the beef is fully heated, remove it from the heat and garnish with sliced green onions and sesame seeds. Serve the Mongolian beef hot over a bed of steamed rice for a complete and satisfying meal.