Combine flour, salt, and pepper and dredge cubed sirloin steak in mixture.
Over medium heat, melt half of the butter and brown meat. Remove from pan and set aside.
Add onions and sweat for a minute. Add mushrooms and remaining butter and cook for another 5 minutes. It’s ok to dash about 2 tbsps of leftover flour to sauce at this point.
Add beef stock and bring to boil.
Combine sour cream, worcestershire sauce, and tomato paste. Adding some beef sauce will help it mix better. Add cooked meat and simmer for about a minute, then add cream mixture.