12 shiitake mushrooms, about 6 ounces total, each 1½ to 2 inches across
Extra-virgin olive oil
1 cup bread crumbs
¼ cup milk
2 pounds ground chuck (80% lean)
1 egg, lightly beaten
2 tablespoons Dijon mustard
1 tablespoon Worcestershire sauce
1 tablespoon minced garlic
1 teaspoon dried thyme
1 teaspoon kosher salt
½ teaspoon finely ground fresh black pepper
⅓ cup barbecue sauce
Prepare the grill for direct and indirect cooking over medium heat (350° to 450°F). Remove and discard the tough stems from the mushrooms and then generously brush them with oil. Brush the cooking grates clean. Grill the mushrooms over direct medium heat, with the lid closed as much as possible, until barely tender, 6 to 7 minutes, turning once or twice. Remove from the grill and, when cool enough to handle, cut the mushrooms into ¼-inch pieces.
In a medium bowl combine the bread crumbs and milk, mix well, and let stand while combining the rest of the ingredients. In a large bowl combine the meat loaf ingredients, including the mushrooms and soaked bread crumbs. Using your hands, mix the ingredients thoroughly but do not overwork. Form into a loaf about 9 inches long and 5 inches wide. Place the loaf in the middle of a large disposable foil pan. Grill over indirect medium heat, with the lid closed, for 45 minutes.
After 45 minutes of grilling time, brush the barbecue sauce over the top and sides of the meat loaf and continue to grill over indirect medium heat, with the lid closed, until the internal temperature reaches 155°F, 40 to 45 minutes more. Remove from the grill and let rest 10 to 15 minutes. Once removed from the grill, the meat loaf will continue to cook, allowing it to reach the recommended 160°F for ground beef. Cut into slices and serve warm.