Step 1: Prepare the Beef

1. Thinly slice the flank steak against the grain into strips about 1/4-inch thick. Slicing against the grain ensures the beef is tender.
2. Pat the beef slices dry with paper towels to remove excess moisture.
3. Toss the beef with cornstarch in a bowl until all pieces are evenly coated. Let the beef sit for 10 minutes while you prepare the sauce. This step helps create a crispy exterior when stir-fried.

Step 2: Make the Sauce

1. In a small bowl, whisk together the soy sauce, brown sugar, water, and hoisin sauce (if using). Set aside.
2. Mince the garlic and grate the ginger. These aromatics will add bold flavor to the sauce.

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, working in batches to avoid overcrowding the pan. Cook for 2-3 minutes on each side until browned and crispy. Remove the cooked beef and set aside on a plate.

Step 4: Sauté the Aromatics

1. Add the remaining tablespoon of oil to the skillet. Reduce the heat to medium and add the minced garlic and grated ginger. Sauté for 30 seconds until fragrant, being careful not to burn the garlic.

Step 5: Combine the Sauce and Beef

1. Pour the prepared sauce into the skillet with the garlic and ginger. Bring it to a gentle simmer, stirring constantly.
2. Return the cooked beef to the skillet and toss to coat evenly in the sauce. Let the mixture simmer for 2-3 minutes until the sauce thickens and clings to the beef.

Step 6: Add Green Onions

1. Stir in the green onions during the final minute of cooking to retain their vibrant color and slightly crunchy texture.

Step 7: Garnish and Serve

1. Transfer the Mongolian Beef to a serving dish and sprinkle with sesame seeds and additional green onions for garnish.
2. Serve immediately over steamed rice, noodles, or alongside sautéed vegetables.