Polish White Borscht (Bialy Barszcz)

Boil kielbasa and 8 cups water in a 6-qt. saucepan. Reduce heat to medium-low; cook to flavor broth, about 25 minutes. Pour liquid and kielbasa into a bowl; reserve. Return saucepan to medium heat. Add butter, garlic, leeks, and onion; cook until soft, about 10 minutes. Add reserved liquid, potatoes, marjoram, and bay leaf; boil. Reduce heat to medium-low; cook until potatoes are tender, about 30 minutes. Discard marjoram and bay leaf; purée soup in a blender. Return soup to pot; bring to a simmer. Meanwhile, whisk sour cream and flour in a bowl, add 1⁄2 cup soup, and whisk until smooth. Pour mixture into soup; cook, stirring, until thickened, about 5 minutes. Cut kielbasa into 1⁄2″-thick slices; add to soup along with horseradish, salt, and pepper. Garnish with dill, parsley, and eggs.