Coat the Steak: In a medium bowl, toss the thinly sliced flank steak with the cornstarch until all pieces are evenly coated.

Combine Sauce Ingredients: In a small bowl, whisk together the brown sugar, minced garlic, red pepper flakes (if using), grated ginger, soy sauce, and water. Set aside.

Heat Oil and Sear Beef: In a large skillet over medium-high heat, add 1½ tablespoons of olive oil. Once hot, add half of the coated beef slices in a single layer. Fry each side for 2-3 minutes, or until they develop a golden-brown crust and a crispy texture. Remove from the skillet and repeat with the remaining oil and beef. Transfer all cooked beef to a plate and set aside.

Add Sauce and Scallions: In the same skillet, pour the prepared sauce and bring it to a simmer over medium heat.

Combine Beef with Sauce: Return the cooked beef to the skillet and toss to coat in the sauce. Let it cook for another 1-2 minutes until the sauce thickens slightly and the beef is fully coated.

Serve Hot: Serve the Mongolian beef over steamed rice or noodles, garnished with extra scallions if desired. Enjoy!