Preheat the oven to 325°F (163°C) and line a baking tray.
In a large mixing bowl, combine 3 cups shredded cooked chicken, 1 1/4 cups shredded mozzarella, 1/4 cup chopped red onion, 1/4 cup chopped cilantro, and 1/2 cup barbecue sauce; stir until evenly mixed.
If the mixture seems dry, add a little more barbecue sauce until it holds together but is not soggy.
Trim the top edge off the baguette and hollow out the center about 1 1/2 inches deep to create a well for the filling.
Spoon the BBQ chicken mixture into the hollowed baguette, pressing gently to fill evenly.
Sprinkle the remaining 1/4 cup shredded mozzarella over the top and drizzle with the remaining 1/4 cup barbecue sauce.
Place the stuffed baguette on the prepared baking tray and bake at 325°F for about 20 minutes, until heated through and cheese is melted.
Remove from the oven, let rest 2–3 minutes, then slice and serve.