Cut the beef against the grain into thin strips about 2 inches long (by 1/4-inch thick). In a large bowl, whisk together the sherry, soy sauce, sesame oil, cornstarch, and sugar. Add the beef and mix well. Set aside.

Preheat a large non-stick skillet over high heat. Add 1 tablespoon of the oil and heat until very hot. Add the scallion whites and beef mixture to the skillet, arrange it in a single layer, and cook, without stirring, until well-browned on the bottom and still pinkish inside, about 1 minute. (It is key to only partially cook the meat at this stage, since it will be finish cooking later in the sauce.) Transfer to a bowl.

Return the skillet to the heat and add the remaining oil. Add the ginger and cook, stirring, until fragrant, about 30 seconds to 1 minute (only a few seconds if using young ginger). Add the scallion greens and tomatoes and cook until slightly softened and saucy, and tomato skin begins to wrinkle, about 1 minute. Return the beef and any collected juice to the skillet, add the hoisin sauce, and cook, stirring, until thick, about 30 seconds more. Season, to taste. Transfer to a serving platter and serve immediately with rice.

Cook's Note: Slicing meat thinly can sometimes be difficult. To make it easier to slice, chill it in the freezer until just firm. Young ginger has a pale golden skin and a bright, new flavor that isn't as assertive as its older self.