Begin by slicing the flank steak against the grain into thin strips. This will ensure that the meat remains tender during cooking.
In a bowl, add the sliced steak and sprinkle with cornstarch. Toss the beef until it is fully coated. This will help create a crispy texture when cooked.
In a large skillet or wok, heat 2 tablespoons of olive oil over medium-high heat. Once hot, add the coated beef in a single layer. You may need to do this in batches to avoid overcrowding the pan.
Cook the beef for about 2-3 minutes on each side until browned and crispy. Remove from the pan and set aside.
In the same pan, add the remaining tablespoon of olive oil. Sauté the minced garlic and grated ginger for about 30 seconds until fragrant.
Add the brown sugar, soy sauce, water, and crushed red pepper flakes (if using) to the pan. Stir well and bring the mixture to a gentle simmer.
Once the sauce is simmering, return the cooked beef to the pan along with the scallions. Toss everything together to ensure the beef is coated in the sauce.
Cook for an additional 2-3 minutes, allowing the sauce to thicken slightly. Serve your Simple Mongolian Beef over a bed of rice or with your favorite vegetables, and enjoy!