In a medium bowl, combine the milk, egg, breadcrumbs, Worcestershire sauce, and 1 tablespoon of barbecue sauce.
On a cutting board, combine the spinach, scallion, and garlic and finely chop. Add the ground beef and turkey, 1 teaspoon of salt, and several grinds of black pepper to the bowl. Use your hands to gently mash the contents together well.
Apply cooking spray all over a 20-inch sheet of heavy-duty aluminum foil. Form the meatloaf mixture into a loaf that measures 9 by 5 inches and is approximately 1 inch thick by piling it onto the foil slightly off the center. To create a sealed packet, fold the long side of the foil over the meatloaf, aligning up the edges, then crimp and fold firmly.
Cook for 10 minutes after placing the package on the grill and covering it with the lid. Utilizing two substantial metal spatulas, flip the packet; cover and cook for a further 10 minutes. Take out and rest for five minutes. Open the envelope slowly (hot steam will escape). The meatloaf needs to be 165 degrees Fahrenheit within and have some grill marks. Apply the final 3 tablespoons of barbecue sauce to the loaf's top and sides. Slices are then served.