Slice the mushrooms and the onions and place in the centre of a greased piece of tinfoil. Place the pork tenderloin on top of the onions and mushrooms and pour over the Quintessential Brown Sauce.
Wrap the foil into a parcel and cook at 180oC/350oF/gas mark 4 for 20–30 mins or until the meat is cooked.
Remove from the oven and open the foil parcel, pour the juices into a pan and add the crème fraîche and just bring to the boil.
Slice the pork and serve with the mushrooms and onions and pour the sauce over the top.