Melt butter in stock pot over medium heat. Cook onions until caramelized and just beginning to brown 7-10 minutes.
Add cabbage, cook for 10 minutes. Add apple, broth, bay leaf, cloves, vinegar, juniper berries, and salt. Bring to a boil, reduce heat to low, cover and simmer for 2 hours, stirring occasionally. Add more broth if needed.
Combine flour and 2 tbls broth until dissolved and stir into Rotkohl. Simmer for another minute. Add more salt and vinegar to taste.