Heat the oil to about 375 degrees (medium hot). The tofu should begin to brown as soon as it hits the oil. Drop the tofu cubes into the oil one by one to keep them from sticking to each other. (See for more about deep-frying.)
When the tofu cubes float to the surface, turn them with long chopsticks to brown them evenly. The surface of the tofu blisters and puffs slightly.
When the cubes are golden brown (about 2 minutes), remove them from the oil and drain them on paper towels. This much can be done well in advance.
Drain the oil from the wok and add 2 tablespoons oil over high heat. Stir-fry the meat just to coat it with the oil, then add the sauce ingredients. Bring the mixture to a boil, then add the scallions. Just before serving, stir in the tofu to heat it through. It loses some of its crispness, but it also absorbs the flavor of the sauce.