Preheat your oven to 375°F (190°C). Spray a baking sheet or loaf pan with nonstick spray.

In a large mixing bowl, combine ground beef, breadcrumbs, eggs, milk, finely chopped onion, minced garlic, Worcestershire sauce, tomato paste, salt, black pepper, dried oregano, and basil. Mix gently until just combined.

Shape half of the meat mixture into a loaf in the center of the baking surface. Create a trench down the center and fill it with mozzarella.

Cover the filled trench with the remaining meat mixture, sealing the edges well to prevent leaking. Shape gently for an even appearance.

In a small bowl, combine ketchup, brown sugar, and balsamic vinegar. Brush this glaze over the top of the meatloaf, reserving some for serving.

Bake the meatloaf in the preheated oven for 50–60 minutes or until the internal temperature reaches 160°F (71°C). Tent with aluminum foil if browning too much.

Let the meatloaf rest for 10 minutes, loosely covered with foil, to allow the juices to redistribute.

Slice the meatloaf and serve with the reserved glaze drizzled on top.