Preheat the oven to 190 C / Gas 5. Pan fry the mushrooms and garlic in a small saucepan for 5 minutes, add salt and pepper and remove from heat. Prepare the pork by slicing the fillet in half crossways. Then with each half cut down the middle lengthways being careful not to cut all the way through. Open out slightly and season with salt and pepper.
Combine the mushroom mix and parsley then spoon evenly onto each pork fillet half. Take a slice of pancetta and wrap around to enclose the pork. You will need 2 or 3 slices per half. Secure with toothpicks.
Heat a frying pan with oil and a knob of butter to medium to high heat. Add the pork fillet and using tongs, seal and brown the pork and pancetta. Remove and place into an ovenproof and hob proof dish; cover with foil.
Bake in the preheated oven until the pork is no longer pink at the centre, about 25 minutes. Check the progress of the pork and place a skewer into the thickest part of the pork to see if cooked. This should be almost cooked through. Remove the foil and return to the oven for 5 to 10 minutes or until cooked through.
Meanwhile, toss the garden lettuce, onion and radish together and whisk oil and vinegar together for dressing.
Remove pork from the oven and set aside on a plate to keep warm. Place the dish onto a hob that is heated to medium/low. Pour in the Sherry and scrape up the meat juices to mix in with a wooden spoon. Cook for a few minutes, season with salt and pepper and finally a knob of butter to gloss and enrich the sauce. The idea is not to have a gravy, merely a drizzle to moisten the meat when poured over.
Plate up the pork and sauce and serve with the salad.