Heat a little butter in a frying pan and add steak.
Fry on medium heat until meat is browned and cooked through. Remove from frying pan.
Add carrot, celery and mushrooms to frying pan and cook on low - medium heat for a few minutes, until tender.
Return meat to pan and add sour cream.
Cook for a few minutes, stirring, until the sour cream has become a light brown sauce and is coating all ingredients.