Heat oil and half of the butter in a saucepan. Add meat strips in batches, stir to seal on all sides. As each batch cooks, transfer to a strainer sitting over a bowl, keeping the juices.
When all meat is cooked, add remaining butter to pan. Add onion and mushrooms, stirring until onion is translucent and mushrooms are soft. Stir in mustard and pepper to taste, and cook briefly. Add a dash of water and scrape up brownings from the bottom of pan. Add sour cream to pan and stir well. Stir in lemon juice, nutmeg and paprika. Season meat with pepper and return to the sauce. Stir through for 2 mins on low heat.
Stir in meat juices to finish sauce. Serve with potatoes and garnish with parsley.