In a medium saucepan, melt the butter. Add the onion and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the apple and cook until the moisture has evaporated, about 3 minutes. Add the bourbon and cook until nearly evaporated, about 3 minutes. Stir in the sauce and bring to a boil. Simmer over low heat for 5 minutes. Use or refrigerate for up to 1 month.