Heat a large oven-safe skillet over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks.
Stir in the chopped onion and minced garlic. Cook for 3 minutes until the onion softens.
Add the tomato sauce, brown sugar, Worcestershire sauce, and mustard. Stir well and let the mixture simmer for 5 minutes.
In a mixing bowl, combine the cornbread mix, egg, and milk. Stir until the batter is just blended.
Pour the cornbread batter evenly over the beef mixture in the skillet. Spread it gently with a spoon.
Transfer the skillet to a 400°F oven. Bake for 15 minutes.
Sprinkle the shredded cheddar cheese over the top. Bake for 5 more minutes until the cheese melts and the cornbread turns golden.