In a large skillet, saute onion in butter for 15 minutes or until lightly browned. , In a large bowl, toss apples with lemon juice. Add the apples, sauerkraut, apple juice, caraway and fennel to the onion. Bring to a boil. Reduce heat; cover and simmer for 15 minutes. Meanwhile, heat sausage according to package directions; cut into slices. Serve with sauerkraut.