Inspired by traditional German flavors, this one-pot meal of knockwurst with braised cabbage and apples is served with spicy mustard.
Melt butter in a large heavy-bottomed pot over medium-low heat. Add onions; cook, stirring occasionally, until very soft and golden brown, about 18 minutes.
Stir in cabbage, apple, cider, vinegar, sugar, salt, and pepper. Cover; cook, stirring occasionally, until cabbage is very soft, about 1 hour.
Tuck sausages into cabbage; cover pot. Cook until heated through, about 20 minutes. Serve with mustard.