Cut the chicken into 1-inch pieces or strips and put into a zip-lock bag with the soy sauce and ginger. Refrigerate for an hour, or overnight. When you’re ready to cook, add the egg white to the bag and squish it around to coat the chicken. Put the cornstarch into a shallow bowl.
To make the sauce, bring all the ingredients to a boil in a medium saucepan; cook for a couple minutes, until thickened.
Heat a couple inches of oil in a heavy pot or skillet set over medium-high heat. When it’s hot but not smoking (a scrap of bread should sizzle when dipped in), dip pieces of chicken in the cornstarch to coat, then gently lower into the hot oil. Cook in batches, then remove from the oil with tongs or a slotted spoon and transfer to paper towels to drain. Pour the lemon sauce overtop and toss to coat; serve immediately, over rice.