Whisk the stock and all but 2 tbsp of the soured cream together. Set aside.
Generously season the steaks. Heat half the butter in a frying pan until sizzling, then cook the steaks for 8 mins, turning them every minute or so, until golden brown on each side. Remove and set aside.
Heat the rest of the butter in the steak pan, tip in the shallots and cook for about 5 mins or until starting to caramelise. Add the mushrooms and cook for a further 5 mins or until golden. Stir in the garlic and paprika along with a pinch of porcini powder and continue to cook for 1 min.
Add the vinegar and leave to simmer for 1 min. Turn the heat down to its lowest setting and pour in the creamy stock, stirring everything together until hot but not simmering, then taste for seasoning and add most of the parsley and the mustard.
Pour in any juices from the steak, before carving it into thick slices and adding to the pan. To serve, top with the remaining soured cream, then scatter with the remaining parsley, a pinch of paprika and porcini powder, if using.