Cut beef into 0,5 inch cubes. Heat the frying pan with oil. Add beef, cook on medium heat for both sides, about 15 minutes.
Add beef to the large pot, add onions, mushrooms and carrots. Mix ingredients very well and cook for about 10 minutes mixing all the time. Add water and bullion cube. Cook on low-medium heat for 2 hours until meat is tender. Add salt and pepper.
In a large pot pour water, add sausages and cook on medium heat for 1 hour. Remove the foam from the top. Remove the sausage from the pot. Cool off sausages a little and remove skin from bratwurst sausages and slice, add to the soup. Add buckwheat, cabbage, carrots, beef bullion, salt and garlic. Cook for 20 minutes until buckwheat and vegetables are cooked. Add kidney beans. Season with pepper and more salt if preferred.
In the frying pan melt butter, add tomato paste cook on low heat for 1 - 2 minutes mixing all the time. Add it to the soup.
Cook soup for 5 - 10 minutes until all ingredients will incorporate well together.