Pat the flank steak slices dry and toss them in cornstarch until lightly coated, ensuring each piece has an even layer for a crisp caramelized texture.

Heat vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the steak in batches and sear until browned, about 2-3 minutes per side.

Remove cooked steak from the skillet and set aside. Carefully wipe out excess oil while leaving flavorful bits stuck to the pan.

Add garlic and ginger to the hot skillet. Sauté for 30 seconds until fragrant, releasing an irresistible aroma that fills the kitchen.

Pour in soy sauce, brown sugar, water, and red pepper flakes if using. Stir and bring to a simmer, creating a glossy, sticky sauce that thickens slightly.

Return the cooked steak to the skillet, tossing gently to coat in the caramelized sauce. Cook for 1-2 minutes, allowing the flavors to meld beautifully.

Add the green onions and toss briefly. They should stay bright and slightly crisp, adding freshness and color to the rich, savory beef.

Serve the Mongolian beef hot over steamed white rice. Enjoy immediately for the best texture, with the sweet-salty glaze perfectly coating each piece.