Cut your loin almost in half, About 1/2 inch from cutting through. Open it up and cover with plastic wrap and flatten it out with a meat mallet till its about 1/4-3/8 of a inch thick. Sautee chopped onion and mushrooms till about cooked through and add some chopped spinach, and let cool. Season your loin good on both sides. Smear some good coarse mustard and cover with sliced swiss cheese. Add your onion mixture and spread evenly. I diced a dill pickle and added to the top. Roll the loin up nice and firm and tie with a few pieces of butcher twine. Season well and let it rest while you get your grill ready for indirect heat.
Cook at 225 with some good smoke, I like apple wood. After about an hour start spraying a little apple juice on it about every 20 minutes. When internal temp reaches 160 take off grill and wrap in foil for a little while and ENJOY!!!