1Boil kielbasa and 8 cups water in a 6-qt. saucepan. Reduce heat to medium-low; cook to flavour broth, about 25 minutes.
3Pour liquid and kielbasa into a bowl; reserve. Return saucepan to medium heat. Add butter, garlic, leeks, and onion; cook until soft, about 10 minutes.
5Add 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.
7Return soup to pot; bring to a simmer. Meanwhile, whisk sour cream and flour in a bowl, add ½ cup soup, and whisk until smooth. Pour mixture into soup; cook, stirring, until thickened, about 5 minutes.
9Cut kielbasa into ½″-thick slices; add to soup along with horseradish, salt, and pepper. Garnish with dill, parsley, and eggs.