Line a 6-quart slow cooker (or a bigger slow cooker) with a large sheet of aluminum foil. Grease with cooking spray and set aside.
In a large mixing bowl combine ground beef, breadcrumbs, onions, garlic, Italian Seasoning, salt, pepper, ground mustard, egg, milk, and Worcestershire sauce. Mix with your hands until combined, but DO NOT overmix.
Divide meat in half.
Press one half of the meat into the bottom of the slow cooker.
Spread shredded mozzarella down the center, leaving about 1/2 inch space around the sides.
Top with remaining meat and shape it into a loaf, and press around to seal the edges. Set aside.
In a small mixing bowl, whisk together the barbecue sauce, ketchup, honey, Worcestershire sauce, and Sriracha sauce.
Brush half of the glaze over the meatloaf. Reserve the remaining half for later.
Cover and cook on LOW for 6 hours or on HIGH for 3 to 4 hours. Meatloaf is done when temperature reaches 160˚F on an Instant Read Thermometer.
Lift out the meatloaf by holding onto the foil; transfer to a baking dish and top with the rest of the glaze.
Pop it under the broiler for 3 to 4 minutes, or until browned.
Remove from oven and let rest for 10 minutes.
Garnish with parsley and serve.