Preheat your oven to 350°F (175°C) and prepare a baking sheet with foil or parchment paper.

In a large mixing bowl, combine the ground beef, diced onion, and breadcrumbs. Pour in the milk and add the eggs, salt, and pepper. Mix gently.

Shape the meat mixture into a loaf on the prepared baking sheet.

In a separate bowl, mix ketchup, brown sugar, and mustard for the glaze. Spread it evenly over the meatloaf.

Bake for 55-70 minutes until the glaze is caramelized and the internal temperature reaches 160°F (71°C).

Remove from oven and let rest for 10 minutes before slicing.