Cut the flank steak with the grain into 2 pieces. Then, cutting against the grain, slice into ¼ inch strips. If the steak is slightly frozen, not solid, it is easier to cut thinly. Use a good, sharp knife.
Place the steak slices in a shallow bowl and add the ginger, cornstarch, 2 teaspoons of soy sauce, 2 teaspoons rice wine or cooking sherry, 1⁄4 teaspoon of the salt and sugar and mix well.
Add 2 teaspoons oil and stir. Set aside. In a small bowl, combine the ketchup, hoisin sauce, Sriracha sauce, the remaining 1 tablespoon of soy sauce and 1 tablespoon rice wine.
Heat a large, flat-bottomed wok over high heat. You know it is hot enough if a tiny drop of water vaporizes within 1 - 2 seconds of contact. Pour in 1 tablespoon of the oil and swirl the wok to distribute the oil.
Add the garlic and stir-fry for 15 seconds or until fragrant. Remove the garlic and discard it. Carefully add the beef strips, spreading it evenly in one layer in the wok.
Be careful not to add more than 12 ounces beef or the wok will cool down and your steak will steam instead of searing, and you will never get good browning.
Cook the beef without stirring until it begins to brown, about 1 minute. Using a metal spatula, stir-fry until beef is lightly browned, about 30 - 60 seconds.
Swirl in the sauce mixture and stir-fry for 30 seconds to 1 minute.
Remove the beef from the wok, but keep the wok on the stove to heat up the beef when ready to serve the sandwich.