Heat the oil in a heavy-based frying pan. Add in the pork medallions, fry for 2-3 minutes on each side. Season with salt and freshly ground pepper and fry until the pork is cooked through, taking care not to over-cook it. Remove the fillets from the pan and keep warm. Add the butter to the pan and melt before adding in the chopped onion and garlic. Sauté for a minute or two. Add the mushrooms to the frying pan and fry until slightly coloured. Add the brandy or sherry to the pan and evaporate the alcohol. Add the cream to the pan and cook briskly until reduced by two-thirds. Add the pork medallions back to the pan, season with salt and freshly ground pepper and mix in the parsley. Heat through before serving with the warm cooked pasta.