Preheat your oven to 375°F (190°C) and line a baking sheet with foil or parchment paper.

In a large mixing bowl, combine the ground beef, breadcrumbs, eggs, milk, onion, garlic, Worcestershire sauce, tomato paste, and seasonings. Mix gently until just combined.

Shape half of the meat mixture into a loaf on the prepared surface or in the loaf pan, making a trench down the center.

Fill the trench with shredded mozzarella, then cover with the remaining meat mixture tightly sealing the edges.

In a small bowl, combine ketchup, brown sugar, and balsamic vinegar. Mix until smooth and brush half of the glaze over the top.

Bake in the preheated oven for 50–60 minutes until golden brown and internal temperature reaches 160°F (71°C).

Let rest for 10 minutes before slicing to allow juices to redistribute.

Serve slices with the reserved glaze drizzled on top, accompanied by your favorite sides.