Coat the Beef: Toss the sliced flank steak in cornstarch, ensuring each piece is coated evenly. Let it sit for 5 minutes.
Prepare the Sauce: In a small bowl, mix soy sauce, brown sugar, water, hoisin sauce, and red pepper flakes. Set aside.
Cook the Beef: Heat vegetable oil in a large skillet or wok over medium-high heat. Add the beef in a single layer and sear for 2-3 minutes per side until crispy. Remove and set aside.
Make the Sauce: In the same pan, add garlic and ginger, sautéing for 30 seconds until fragrant. Pour in the sauce mixture and let it simmer for 2 minutes.
Combine Everything: Return the cooked beef to the skillet and toss to coat in the sauce. Cook for another 1-2 minutes until the sauce thickens.
Serve: Garnish with green onions and sesame seeds. Serve hot over white rice.