Heat a heavy-bottomed pot over medium-high heat. Melt 1 tablespoon of butter in the pot, then add the onion. Stir well and cook until the onion is tender and lightly browned.
Add the diced tomatoes and tomato paste, stirring for 2 minutes until the juices are released. Then, add the chopped cabbage, carrot, and potato chunks, and stir-fry the vegetables for 2 to 3 minutes. Next, add the beef broth, turn the heat up to high, and bring the soup to a boil.
Once the soup reaches a boil, reduce the heat to medium and add the beef sausage. Cover and simmer for 20 minutes, or until the potatoes are soft. Stir the soup occasionally to prevent the potatoes from sticking to the bottom.
Meanwhile, melt the remaining butter in a clean frying pan over medium heat. Add the all-purpose flour to the pan and stir for a few minutes until lightly browned. Turn off the heat, transfer to a small bowl, and mix well with 1 cup of water.
Once the potatoes are soft, stir in the all-purpose flour slurry to thicken the soup. Add salt and ground black pepper to taste. Serve immediately.