In a skillet, sauté the diced onion and minced garlic until softened. Set aside to cool.
In a large bowl, combine ground beef, crushed stuffing mix, eggs, milk, Cajun seasoning, Worcestershire sauce, and the sautéed onions and garlic. Mix until well combined.
Transfer the meat mixture to a baking dish and shape it into a loaf.
In a small bowl, mix ketchup, barbecue sauce, brown sugar, and mustard (if using). Spread half of the sauce over the meatloaf. Reserve the rest for dipping.
Bake at 375°F for about 1 hour, or until the internal temperature reaches 160°F.