Method:
Cut the pork tenderloin into strips, or get your butcher to do it.
Pu a large pan on a medium high heat and add a small amount of oil. Season the pork well with salt and pepper and fry in the pan to get some colour. When browned, remove from the pan and put to one side.
Add the mushrooms to the pan, once they are browned add the onions and lower to a medium heat and cook until soft.