Place the pork loin joint on a clean work surface. Firmly place your hand on top of the joint, and with a sharp knife, start slicing the loin lengthways, about 1 to 2cm from the bottom of the joint. Slice, following the curve of the joint slightly, until you've almost reached the side. Lay the joint flat and make one more slice at the thick end of the joint, so that you are able to fold it over to make one flat rectangle.
With a mallet, pound the pork to an even thickness. Season with salt and pepper and set aside.
In a large frying pan, cook the chopped bacon till it has released its fat. Add the onions and mushrooms and cook till soft and translucent. Add 100g of the chopped walnuts and cook a further 5 minutes.
Preheat the oven to 180 degrees C.
Spread the filling over the flattened pork loin. From a short end, roll up as you would a Swiss roll. Tie securely with kitchen string.
Using a mortar and pestle, crush together the remaining walnuts and the coloured peppercorns. Use this mixture to cover the outside of the roulade, generously covering all sides.
Heat the oil in a frying pan. Add the roulade and brown on all sides. Transfer to a roasting tin (preferably set on top of a roasting rack) and spread the top of the roulade with butter.
Roast the roulade in the preheated oven for 1 hour.