Make the marinade. In a 13 x 9 casserole dish, whisk together the dry sherry, soy sauce, ketchup, garlic, ginger paste, and sesame oil. Then, add the flank steak to the dish, turning to coat both sides. Marinate at room temperature for 2 hours, turning the steak every 30 minutes to make sure even marination.
Boil the marinade. After marinating, pour the leftover marinade into a small saucepan. Bring it to a rolling boil for 3 minutes to make it safe for sauce. Set it aside for serving. Meanwhile, preheat the broiler to high.
Broil the steak. Place the marinated flank steak on a baking sheet. Then, broil the steak for 4 minutes per side for medium-rare doneness. Next, adjust the cooking time as needed for your desired doneness. Use tongs to transfer the steak to a platter and let it rest for 15 minutes to allow the juices to redistribute.
Serve. Slice the steak thinly against the grain on a cutting board. Serve with the boiled marinade as a sauce. You can reheat and strain the marinade for a smoother sauce before serving.