Melt the butter in a Dutch oven over medium-high heat and cook the onions until lcaramelized and just beginning to brown, 7-10 minutes. Add the cabbage and cook for 5 minutes. Add the apple, broth, bay leaf, cloves, juniper berries, red currant jam, red wine vinegar, sugar and salt. Bring to a boil, reduce the heat to low, cover and simmer for 2 hours, stirring occasionally. Add more broth if needed. Combine the flour and water until dissolved and stir into the Rotkohl. Simmer for another minute. Add more salt, sugar and vinegar to taste.
Traditionally served with roasts, Rouladen, Sauerbraten and potatoes, Spaetzle or Knoedel.