Combine meats, onion, egg, milk, salt, and pepper in a large bowl.
Place meat mixture into a lightly greased 5x9 inch loaf pan.
Bake for 30 minutes.
While the meatloaf is baking, combine brown sugar, Worcestershire sauce, and ketchup in a small bowl.
After 30 minutes, pour half of the ketchup mixture evenly over the meatloaf, and bake until internal temperature reaches 165 degrees F/75 degrees C (about another 30 minutes).
Allow meatloaf to sit for 10 minutes before slicing and serving.
Use the remaining ketchup mixture for additional topping or as a dipping sauce.