Preheat oven 350ºF. Cut boule in a crosshatch pattern, almost to the bottom of the loaf, without cutting all the way through, and place on a baking sheet.
In a small bowl, combine melted butter, Parmesan cheese and 1 tbsp. garlic and whisk until well-combined. Brush the mixture generously over the top of the sliced boule.
Heat oil in a medium skillet over medium heat. Combine 1 tbsp. garlic, red onion and mushrooms and sauté for approximately 2 minutes. Add cooked, shredded chicken and barbecue sauce and stir until chicken is completely covered.
Using a spoon, stuff the chicken mixture into the cuts in the boule, ensuring every cut is filled. Add Mozzarella slices between the cuts of the boule. Sprinkle parsley on top and cover boule with foil.
Transfer baking sheet to oven, and bake boule until cheese is melted (approximately 5-10 minutes). Remove the foil and bake uncovered for an additional 5 minutes before serving.
