Firstly, heat the oil or butter in a large heavy-based saucepan. Add the beetroot, carrot, celery, potato, onion and garlic, then sauté for a couple of minutes until the veg are well combined and coated with the fat. Add the stock and season with salt and black pepper.
Secondly, bring the soup almost to the boil, then cover and simmer for about 15 minutes. Add the cabbage and tomatoes. Cover the pan and leave the soup to simmer for another 20 minutes. Taste for seasoning and add more salt and pepper if necessary.
Thirdly, trim the fat off the steak. Heat a griddle pan until it is smoking and cook the steak for just a couple of minutes on each side. Remove the steak and leave it to rest for 5 minutes, then slice it as thinly as you can and add any meat juices to the soup.
To finish, divide the strips of steak between the soup bowls and ladle the soup on top. Add dollops of soured cream or crème fraîche, if using, and a sprinkling of dill.
