First, take a large zip-top bag and add the sliced flank steak along with the cornstarch. Seal the bag tightly and shake to coat the beef evenly. Set it aside while we prepare the sauce.
Next, grab a medium saucepan and add 1 tablespoon of olive oil and 1 tablespoon of sesame oil. Toss in the minced garlic and ginger, and heat over medium-high heat for about 1 minute. Be careful not to let the garlic scorch, as it will turn bitter.
Once fragrant, add the soy sauce, water, and brown sugar into the pan. Stir well until the sugar has dissolved completely. Bring the mixture to a boil and let it bubble for about 2 to 3 minutes.
Reduce the heat to low and let the sauce simmer for about 5 minutes. This will allow it to thicken slightly. Taste the sauce and adjust flavors if necessary—maybe a little more sugar or soy sauce.
While the sauce simmers, heat a large skillet over medium-high heat and add the remaining olive oil and sesame oil.
Introduce the coated flank steak into the skillet. Cook for about 5 to 7 minutes, flipping the beef pieces intermittently. You want them to be cooked through but still tender.
Once the beef is cooked, pour in the soy sauce mixture. It will bubble up and create a deliciously sticky glaze. Toss the beef to coat evenly.
Let the sauce simmer for an additional 3 minutes on medium-high heat. Then reduce to low, allowing it to simmer for another 3 to 5 minutes until the sauce thickens to your liking.
Finally, give everything a good stir to ensure all pieces of beef are nicely coated. Garnish with the sliced green onions and sesame seeds, if using, then serve immediately. Enjoy your homemade Mongolian Beef while it’s warm!