Slices of spicy kielbasa enliven this thick, hearty soup. Serve with warm pumpernickel bread.
1. Heat the oil and butter in a Dutch oven or large soup pot until the butter is melted. Add the onions and potatoes and toss to coat evenly. Sprinkle on the sugar, salt, and pepper and cover the pan. Cook slowly for 15 to 20 minutes or until the potatoes are tender. Scoop out half the potato mixture and place it in the container of a blender or processor to cool.
2. Pour the water into the soup pot, and add the cabbage. Prick the kielbasa in several places and drop it into the liquid. Stir in the thyme and allspice. Cover the pan and cook slowly for 20 to 30 minutes or until the cabbage is tender and the kielbasa is firm. Remove the kielbasa and peel it if you wish. Slice it into ½-inch-thick rounds and return the pieces to the soup.
3. Ladle about 1 cup of the broth into the potato mixture in the blender or processor and whirl until smooth. Gradually stir it into the soup. Cook, uncovered, for 10 minutes, and serve piping hot.