01 - Set the oven to 350°F (175°C) and allow it to reach temperature while preparing other components.
02 - In a large mixing bowl, combine ground beef, chopped onion, breadcrumbs, eggs, milk, Worcestershire sauce, salt, and black pepper. Blend until just incorporated; do not overwork the mixture.
03 - Transfer the mixture to a lightly greased loaf pan or shape into a loaf on a parchment-lined baking sheet. Press gently to form an even thickness.
04 - In a small bowl, blend ketchup, brown sugar, and apple cider vinegar until smooth.
05 - Spread the glaze evenly over the top surface of the formed loaf.
06 - Position the loaf in the oven and bake for 60 to 75 minutes, until cooked through and an internal temperature of 160°F (71°C) is achieved.
07 - Remove meatloaf from the oven and allow it to rest for 10 minutes before slicing and serving.