Place the thinly sliced flank steak into a large bowl. Add the cornstarch and toss until every piece of beef is thoroughly coated.

In a small bowl, whisk together the soy sauce, water, and brown sugar until the sugar begins to dissolve. Set aside.

Heat 2 tablespoons of vegetable oil in a large wok or skillet over medium-high heat. Add the cornstarch-coated beef in a single layer. Cook for 2 to 3 minutes per side until deeply browned and crispy. Remove the beef from the skillet and set aside on a plate.

In the same skillet, add the remaining 1 tablespoon of vegetable oil. Add the sliced yellow onion, minced ginger, and minced garlic. Stir-fry for 2 minutes until the onions are slightly softened and fragrant.

Pour the soy sauce mixture into the skillet. Bring the liquid to a simmer and let it cook for 3 to 4 minutes until the sauce thickens and becomes syrupy.

Return the cooked beef to the skillet. Toss continuously for 1 to 2 minutes, ensuring all the meat is evenly coated in the sticky glaze.

Remove the skillet from the heat. Stir in the green onions, fresh cilantro, and the fresh lime juice.

Serve the Mongolian beef immediately over a bed of hot cooked white rice. Garnish with sesame seeds and a drizzle of chili oil.