Remove any membrane from the pork fillet and discard. Cut into thin strips about 1x6cm/½x2½in. Season the meat with salt and pepper.
Heat a large non stick frying pan until hot, add a little oil when very hot, then add the meat. Stir fry really briskly until brown, if cooked really fast there should be no juices, but take care not to overcook. Lift out with a slotted spoon onto a plate.
Put the butter in the pan, add the onions and cook over a low heat until the onions are really tender. Sprinkle in the paprika and stir in the mushrooms. Turn up the heat and toss for a minute.
Add the pork and cream to the pan and heat through. Add the lemon juice and season with salt and pepper.
Garnish with the parsley and serve the mashed potato alongside.