Place the pork loin onto a chopping board and using a sharp knife make a deep incision along the length of the loin, taking care not to cut all the way through. Open the loin like a book then beat lightly with a meat hammer. Rub the loin with thyme, garlic and seasoning.
For the mushroom stuffing:
In a frying pan, melt the butter and cook and stir the onion until translucent. Add mushrooms and cook for a further 5 minutes. Remove from the heat and set aside to cool. Stir in breadcrumbs, cheese and parsley and mix well. Season with salt and pepper.
Spread the mushroom mixture evenly over the pork loin, then roll the loin up tightly and secure with skewers. Wrap the loin with the bacon and transfer to a baking dish. Add a splash of water to the dish.
Roast for 1 hour until the pork is no longer pink in the centre. Allow to rest for 15 minutes before slicing and serving.