Special equipment: Heavy-duty foil
Prepare a grill for medium-high heat.
Stir together the milk, egg, breadcrumbs, Worcestershire sauce and 1 tablespoon of the barbecue sauce in a medium bowl.
Finely chop the spinach, scallion and garlic together on a cutting board. Transfer to the bowl, along with the ground beef and turkey, 1 teaspoon salt and several grinds of black pepper. Gently combine the mixture well with your hands.
Coat a 20-inch piece of heavy duty aluminum foil all over with cooking spray. Pile the meatloaf mixture onto the foil just off the center and form into a 9-by-5-inch loaf that’s about 1-inch thick. Fold the long half of the foil over the meatloaf, lining up the edges, and crimp and fold tightly to make a sealed packet.
Put the packet on the grill, close the grill lid and cook for 10 minutes. Turn the packet over using 2 large metal spatulas; cover and grill 10 minutes more. Remove and let rest for 5 minutes. Carefully open the packet (hot steam will escape). The meatloaf should have some grill marks and an internal temperature of 165 degrees F. Brush the top and sides of the loaf with the remaining 3 tablespoons barbecue sauce. Cut into slices and serve.