The finer the zest the better in this fast recipe. Use the smallest holes of a box grater, or zest into bigger strips then chop with a knife.
In a large bowl, mix together the soy sauce and sherry. Add the chicken and stir to coat. Marinate for at least 15 minutes.
Heat a flat-bottomed wok or large skillet over high heat until almost smoking. Add the oil to the pan and swirl to coat the pan. Immediately add the scallions and a large pinch of salt. Cook, stirring constantly, until the scallions wilt and smell fragrant, about 30 seconds. Add the garlic and ginger and stir constantly for 10 seconds. Add the chicken and any leftover marinade and cook, stirring constantly, until the chicken is cooked through, about 1 minute. Turn off the heat and add the orange zest and sesame oil. Stir then serve.