Preheat your oven to 350°F (175°C) and lightly grease a 9x13 baking dish with nonstick spray.

In a large mixing bowl, combine ground beef, finely minced onion, eggs, bread crumbs, Worcestershire sauce, pepper, seasoning salt, garlic powder, and ½ cup of ketchup. Mix gently until just combined.

Take half of the meat mixture and press it evenly into the prepared baking dish, forming a compact base layer.

Layer fresh mozzarella slices on top of the meat base, ensuring even distribution.

Cover the mozzarella with the remaining meat mixture, sealing all edges tightly.

In a small bowl, mix ¼ cup of ketchup with brown sugar and mustard until well combined. Spread it evenly over the top.

Place the baking dish in the preheated oven and bake for 1 hour and 15-30 minutes, checking that the internal temperature reaches 160°F (70°C).

Once baked, remove the meatloaf from the oven and allow it to rest for several minutes before slicing.