Take steaks out of the refrigerator about 30 minutes before processing. In the meantime, peel and wash the sweet potatoes and cut them into cubes of about 1 cm. Mix with 1 teaspoon of salt and sugar each and leave to stand for about 30 minutes.
Cut the fat off the steaks. Dab the steaks dry and rub them all around with 1 teaspoon each of salt and sugar. Let the meat rest for about 10 minutes. Put spiced sweet potato cubes and cream in a pot, bring to the boil and cook over medium heat for 15-20 minutes until tender, stirring occasionally.
Heat the oil in a large frying pan bit by bit. Fry the steaks in it in portions on each side for about 2 minutes. Take them out and let them rest wrapped in aluminium foil. Add butter to the sweet potatoes and mash everything in the cream to form a puree. Season to taste with salt. Arrange meat and puree with 1 tablespoon of sour cream each on plates.
Boil in hot water any leftover meat and vegetables that have to be removed and left lying around. This creates a wonderful broth, which only needs to be salted.