Step 1: Toss sliced flank steak with cornstarch until fully coated. Let sit for 10 minutes while you prep other ingredients.

Step 2: In a small bowl, whisk together soy sauce, water, brown sugar, ginger, and red pepper flakes to make the sauce.

Step 3: Heat 2 tbsp oil in a large wok or skillet over high heat. Add beef in a single layer (work in batches if needed) and cook 1-2 minutes per side until crispy. Remove and set aside.

Step 4: Reduce heat to medium. Add remaining 1 tbsp oil and sauté garlic for 30 seconds until fragrant.

Step 5: Pour in the sauce mixture and bring to a simmer. Cook 2-3 minutes until slightly thickened.

Step 6: Return beef to the pan along with green onions. Toss everything together for 1 minute until well coated and heated through.

Step 7: Serve immediately over steamed rice.