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.
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.