01 - 
                Whisk together soy sauce, lemon juice or rice vinegar, and sugar in a medium bowl until the sugar dissolves.
              
              
              
                02 - 
                Add sliced flank steak to the marinade, ensuring all pieces are coated. Set aside to marinate while preparing other ingredients.
              
              
              
                03 - 
                Heat a small amount of oil in a large nonstick frying pan over high heat. Add the sliced ginger and onion. Reduce to medium heat and stir-fry until the onion softens and becomes translucent.
              
              
              
                04 - 
                Push the onion and ginger to the side of the pan. Increase heat to high, add marinated beef in a single layer (reserve marinade). Let cook undisturbed until seared, then flip to sear the other side.
              
              
              
                05 - 
                Mix cornstarch with 80 ml water and stir into the reserved marinade until well combined.
              
              
              
                06 - 
                Pour the sauce into the pan. Toss all ingredients together and cook until the beef is just cooked through and the sauce thickens to a glossy consistency. Season generously with freshly ground black pepper before serving.