Take 1 c liquid (saving the rest for another use). Bring to a boil and reduce by half. Add soy sauce, sherry, garlic, and vinegar. Mix in cornstarch mixed with water. Cook until thickened. Taste for seasoning, correcting with salt as necessary. Add scallions at the last moment. Pour sauce over chicken and flick a few drops of sesame oil over the top. Serve warm or cool.