Melt the butter in a large saucepan. Add the onion and cook over moderate heat, stirring, until softened, about 4 minutes. Add the apples, ginger, sage, vinegar, sugar and 1/4 cup of water and bring to a boil. Cover and cook over low heat until the apples are very tender, stirring and mashing occasionally, about 10 minutes. Season the compote with salt.