First Step: PREPARE THE BEEF

1. Slice the Beef: Begin by slicing the flank steak or sirloin against the grain into thin strips, about ¼ inch thick. This will ensure the beef is tender and easy to chew once cooked. To make slicing easier, you can freeze the beef for about 30 minutes before cutting, which helps it firm up.
2. Coat the Beef with Cornstarch: In a small bowl, toss the sliced beef with cornstarch. This will create a light coating that helps the beef crisp up and absorb the sauce, enhancing both the texture and flavor.

Second Step: MAKE THE SAUCE

1. Combine the Sauce Ingredients: In a medium bowl, whisk together the soy sauce, water, brown sugar, hoisin sauce, garlic, ginger, red pepper flakes (if using), and rice vinegar. Stir until the brown sugar dissolves completely, and the sauce is smooth. Set the sauce aside.

Third Step: COOK THE BEEF

1. Heat the Oil: In a large skillet or wok, heat the vegetable oil over medium-high heat. When the oil is hot and shimmering, add the beef in a single layer. Work in batches if necessary to avoid overcrowding the pan, which can prevent the beef from searing properly.
2. Stir-Fry the Beef: Cook the beef for about 2-3 minutes, flipping it occasionally, until it is browned and crispy on the outside but still tender on the inside. Be careful not to overcook the beef, as it can become tough.
3. Remove the Beef: Once the beef is cooked, remove it from the skillet and set it aside on a plate. The beef will continue to cook in the sauce later, so don’t worry if it’s slightly underdone.

Fourth Step: MAKE THE MONGOLIAN BEEF SAUCE

1. Cook the Sauce: In the same skillet, pour in the prepared sauce. Bring it to a simmer over medium heat. Stir occasionally, scraping up any browned bits from the bottom of the pan. These bits add incredible flavor to the sauce.
2. Simmer the Sauce: Allow the sauce to simmer for about 3-4 minutes, or until it thickens slightly. The sugar in the sauce will caramelize, creating a glossy, rich texture.

Fifth Step: COMBINE THE BEEF AND SAUCE

1. Return the Beef to the Pan: Once the sauce has thickened, add the cooked beef back into the skillet, tossing it gently to coat the beef in the sauce. Let it cook for another 1-2 minutes, allowing the beef to absorb the flavors of the sauce.
2. Add the Green Onions: Stir in the sliced green onions, reserving a small handful for garnish. Cook for another 30 seconds until the green onions are tender and fragrant, but still vibrant green.

Final Step: GARNISH AND SERVE

1. Garnish: Transfer the Mongolian Beef to a serving platter. Garnish with the remaining green onions and sesame seeds, if desired.
2. Serve: Serve the Mongolian Beef over steamed jasmine rice, or alongside stir-fried vegetables, noodles, or even a simple cucumber salad for a complete meal.