Step 1: Prepare the Beef : Slice the beef thinly against the grain (about 1/4 inch thick). Pat dry with paper towels to remove excess moisture. Toss beef slices in cornstarch until evenly coated. Shake off excess (This helps create a crisp exterior and thickens the sauce later.)

Step 2: Make the Sauce : In a small bowl, whisk together soy sauce, brown sugar, water, hoisin sauce (if using), garlic, ginger, and red pepper flakes. Set aside.

Step 3: Sear the Beef : Heat 1 tbsp oil in a large skillet or wok over high heat. Add half of the beef and sear for 1–2 minutes per side until browned and just cooked. Remove to a plate and repeat with remaining beef, adding more oil if needed.

Step 4: Combine and Glaze : Reduce heat to medium. Pour the sauce mixture into the same pan and simmer for 2–3 minutes until it thickens slightly. Return the cooked beef to the pan and toss to coat evenly in the sauce. Add scallions and cook for another 1 minute — just until wilted but still bright green.

Step 5: Serve : Spoon the Mongolian Beef over steamed jasmine rice. Garnish with extra scallions or sesame seeds for presentation.