Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine ground beef, breadcrumbs, chopped onion, beaten eggs, milk, and seasoning mix.
Mix well until all ingredients are thoroughly combined.
Transfer the meat mixture into a loaf pan and shape it into a loaf.
Place the loaf pan on a baking sheet to catch any drippings.
In a small bowl, mix together ketchup, brown sugar, and mustard for the glaze.
Spread the glaze evenly over the top of the meatloaf.
Cover the meatloaf with aluminum foil and bake for 1 hour.
Remove the foil and bake for an additional 15 minutes to brown the top.
Let the meatloaf rest for 10 minutes before slicing and serving.