Fry the onion in the butter in a large frying pan until soft. Stir in the garlic and mushrooms and fry over high heat until browned. Add the paprika, fry for 30 seconds, then tip into a bowl. Heat the vegetable oil in the same pan, and cook the steaks for 2-3 minutes each side; keep warm. Return mushrooms to pan, pour in the beef stock and bubble fiercely to reduce by two-thirds.
Take off the heat and stir in a the soured cream, parsley and salt and pepper. Serve with tagliatelle.