Begin by cutting the meat into three equal portions. Slice each
portion against the grain* into 2-inch-long slices, ½ inch thick. Set
aside.
In a bowl, make a marinade by combining the cornstarch,
sherry, soy sauce, ginger, garlic, the ¼ teaspoon sugar, and the salt.
Stir until the cornstarch is no longer visible and then stir in the
meat. (Do this just before you stir-fry; no need to let it sit for
long.)
Heat the wok and flick water in to see if it’s ready: the water
will evaporate instantly if it’s hot enough. Swirl in the oil and then
add the beef, spreading it out so it all gets a sear. Don’t touch it for
1 minute. Give the meat a stir to sear the other side, and when it’s
just brown but not fully cooked, remove it to a plate.
Add the tomatoes, oyster sauce, chicken stock, the ½ teaspoon
sugar, and the scallions. Cover the pan for 20 seconds to intensify
the flavor.
Lift the lid, add the beef, and cook for another 30 seconds to a
minute. When the beef is cooked through and the tomatoes are still
somewhat firm but have released all their juices, remove to a
platter and serve.
* To season your wok, put it on low heat and heat for 1 to 2
minutes, until the water has evaporated. The wok may change
color. Turn off the heat and let the wok cool. Then, as a final
step, turn up the heat to high (test the heat by flicking in water;
when it evaporates, it’s ready), swirl in 2 tablespoons of peanut