Sauté onions and mushrooms with butter, salt, pepper and a dash or two of Worcestershire sauce until golden. Allow to cool.
Separate strands of ground pork and ground beef in a large bowl. Add finely diced onion and celery, bread crumbs, beaten eggs, mustard, garlic powder, thyme, dashes Worcestershire sauce, salt and pepper. Mix gently but thoroughly (best to use your hands to mix)
Flatten meatloaf mixture onto wax paper covered baking sheet to about 1/4 to 3/8 inch thick in a 9 inch by 9 inch rectangle.
Layer provolone cheese on top of meatloaf mixture, followed by sautéed onion and mushroom mixture. Leave space around three edges to seal.
Carefully roll the meatloaf into a roll, using the wax paper to help roll it. Seal the ends by pressing gently.
Cook indirect on Weber kettle grill with a wood chunk added to the charcoal for smoke at 300 to 400 deg until internal temperature reaches 165 degrees.
Slice into 1/2 inch thick pinwheels and serve.