In a medium bowl combine brown sugar, vinegar, oyster sauce, sesame oil, and ¼ cup soy sauce.
Heat vegetable oil in a large skillet over medium-high heat. Season chicken with salt and pepper. Add to the skillet and cook while stir-frying for 5-7 minutes or until cooked through. Add garlic and dry sherry. Add the sauce mixture and bring to a boil. In a small bowl mix together cornstarch and ¼ cup soy sauce. Stir into the boiling sauce in the skillet. Add cashews and ¼ cup water. Cook for an additional minute.
Top with green onions for serving.