Heat a wok or saucepan over high heat and add the oil. When the oil is hot, add the ginger and garlic. Stir-fry just until fragrant, and then add the juice, vinegar, brown sugar and soy sauce. Bring to a boil and cook until the sugar has dissolved. Stir in the cornstarch and water mixture and cook until thickened, about 1 minute. Stir into any nearly completed stir-fry dish and bring to a boil until the sauce thickens, about 1 minute.