In large bowl, gently combine ground pork, bread crumbs, garlic, egg, Worcestershire sauce, mustard, salt and pepper; don not overtax. Set aside.
Place prepared spinach in large bowl.
In small nonstick skillet, melt butter over medium-high heat. Sauté mushrooms and onion until onion is translucent and mushrooms begin to soften, 3-5 minutes. Remove from heat and allow mixture to cool, about 10 minutes.
Meanwhile, transfer mushroom mixture to bowl with spinach. Add breadcrumbs, egg, parsley, marjoram, slat, pepper and nutmeg. Stir to combine.
Place meat mixture between two sheets of wax paper and roll into a rectangle about 15 x 8 inches. Remove top sheet. Spread spinach-mushroom mixture evenly over meat, leaving 1/2 inch border. Roll up meat from the short end in a jelly-roll style, lifting and removing paper as you go. Pinch ends of meatloaf and bottom along seam to contain filling.
Place meatloaf into a 8 x 4-inch lightly greased loaf pan. Bake for about 1 1/4 – 1 1/2 hours or until instant-read thermometer into centre of meatloaf registers 160°F. Top of meatloaf should be nicely browned.
Remove meatloaf from oven. Transfer to a clean cutting board and slice into 1/4-inch thick slices.