Get prepared: Peel and chop 1 medium onion. Cut 4 ounces smoked sausage into slices. If the sausage is in an artificial casing, peel it first. Use a peeler to remove the skin from 3 medium potatoes and cut them into pieces about 1 inch (2.5 cm) thick. Cube the 3 ½ ounces smoked bacon as well.
Cook sauerkraut: Place 1 pound sauerkraut in a larger saucepan with its juices and pour 1 cup water over it. Add ½ teaspoon caraway seeds, 2 bay leaves, and 8 allspice berries. Cover and bring to a boil, then reduce the heat to one-third. Simmer gently for about 20 minutes so that the kraut is tender.
Meanwhile, pour 3 cups water into a smaller saucepan. Add 1 teaspoon salt, ½ teaspoon caraway seeds, and the diced potatoes. Bring to a boil, then reduce the heat to one-third. Cover and cook gently for about 10 minutes, until the potatoes are almost tender.
Heat 1 Tablespoon lard in a skillet over medium-high heat. Add the chopped onion and fry for about 5 minutes, stirring constantly. Add the sausage slices and bacon cubes. Cook for another 5 minutes until golden brown. Set aside.
When the potatoes are almost tender, add them to the saucepan with the cooked kraut, along with the water in which they were cooked.
Whisk 1 Tablespoon all-purpose flour in ¼ cup milk (lukewarm) so that there are no lumps. Stir the milk and flour mixture into the soup.
Add fried onions, sausage and bacon. Mix well. Cover and simmer over low heat for about 15 minutes. Stir from time to time as the flour tends to settle to the bottom of the saucepan and form lumps.
Finally, remove the soup from the heat source and stir in ¾ cup sour cream. Do not cook anymore. Season to taste with salt and possibly ground black pepper.