Preheat your oven to 350°F (175°C). Lightly grease a 9x5-inch loaf pan or line a baking sheet with parchment paper if you prefer a free-form loaf. Preparing the pan ensures easy removal and prevents sticking.

In a large mixing bowl, whisk together the milk and the large egg until they are well combined. This liquid base will help to moisten the breadcrumbs and act as a binder for the meatloaf, keeping it tender.

To the milk and egg mixture, add the finely chopped yellow onion, minced garlic, Worcestershire sauce, 1/2 cup of ketchup, 1 teaspoon of Dijon mustard, salt, black pepper, and dried parsley (if you're using it). Mix everything thoroughly with a spoon or whisk to ensure all the flavors are evenly distributed throughout the liquid base.

Add the plain dry breadcrumbs to the wet mixture and stir until they are fully moistened and have absorbed some of the liquid. Then, add the 2 pounds of ground beef (80/20 lean). Using your hands, gently mix all the ingredients until just combined. Be careful not to overmix, as this can compress the meat and make the meatloaf tough and dry.

Transfer the meatloaf mixture to the prepared loaf pan, pressing it gently and evenly into the pan to ensure a consistent shape. If you're using a baking sheet, shape the mixture into an oblong loaf, approximately 9x5 inches, directly on the parchment paper. This free-form method allows for more surface area for the glaze to caramelize.

In a small bowl, whisk together the remaining 1/2 cup of ketchup, 2 tablespoons of packed brown sugar, 1 tablespoon of apple cider vinegar, and 1 teaspoon of Dijon mustard until the mixture is smooth. This combination creates the signature tangy-sweet glaze that will add a burst of flavor to your meatloaf.

Place the meatloaf in the preheated oven and bake for 45 minutes. This initial baking period allows the meatloaf to set and begin cooking through before the glaze is applied, preventing the glaze from burning too quickly.

After 45 minutes, carefully remove the meatloaf from the oven. Evenly spread the prepared glaze over the entire top of the meatloaf. Return it to the oven and continue baking for another 15-25 minutes, or until the internal temperature reaches 160°F (71°C) when measured with a meat thermometer inserted into the thickest part. The glaze should appear slightly caramelized and glossy.

Once cooked, remove the meatloaf from the oven and let it rest in the pan for 10-15 minutes before slicing. This resting period is essential as it allows the juices to redistribute throughout the loaf, resulting in a more moist and flavorful meatloaf. After resting, carefully slice and serve warm.