Prepare pork loin by peeling off the silverskin and use a fillet knife to trim away any excess fat.
Dry brine the loin by applying kosher salt all over and rub it in. Place loin in the refrigerator and leave for 2-3 hours.
Prepare stuffing by heating olive oil in a skillet over medium heat. Add chopped onion and saute. Add chopped mushrooms and cook for another 3-5 minutes. Remove from heat and mix in rosemary and oregano. Put to one side to cool.
Butterfly the pork loin by cutting it lengthwise along its side. Cut to within 1/2 inch to its other side to keep the loin intact. Open the loin in a book-like fashion.
With the loin rolled out, place the stuffing in the meat. Aim to level it from edge to edge.
Roll the meat over the stuffing, creating a log shape. Tie with twine, tying every 1-1 1/2 inch.
Coat the rolled loin in dry rub.
Fire up your smoker to 225°F (107°C). If you are using a charcoal grill, set it up for 2-zone cooking. Add your chosen wood and a full water pan.
Place loin on smoker grates. Smoke for 2-3 hours, until the internal temperature is about 135°F. Time will vary depending on meat quantity and stuffing ingredients, so take a reading after 2 hours.
Remove loin from smoker. Slice into 1/2 inch thick slices and serve immediately.