Start by slicing the flank steak against the grain into thin strips, about 1/4 inch thick. Place the beef in a bowl and sprinkle with cornstarch, tossing until evenly coated.
In a large skillet or wok, heat 2 tablespoons of olive oil over medium-high heat. Add the cornstarch-coated beef and cook for about 3-4 minutes, stirring frequently, until browned and just cooked through. Remove from the skillet and set aside.
Add the remaining tablespoon of olive oil to the skillet. Stir in the minced garlic and grated ginger, cooking for about 30 seconds until fragrant.
Add the brown sugar, soy sauce, water, and optional red pepper flakes to the skillet. Stir to combine and bring to a simmer, allowing it to thicken slightly, about 2-3 minutes.
Return the cooked beef to the skillet, tossing to coat in the sauce. Add the scallions and cook for an additional minute.
Serve your Simple Mongolian Beef over steamed rice or noodles. Garnish with additional scallions if desired.
