Step 1: Preheat the Oven : Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or lightly grease a loaf pan.
Step 2: Make the Meatloaf Mixture : In a large mixing bowl, combine the milk, eggs, Worcestershire sauce, and ketchup. Add breadcrumbs and stir to moisten them. Let them soak for 2–3 minutes. Add chopped onion, salt, pepper, garlic powder, and parsley. Add the ground beef and gently mix using your hands or a spatula. Don’t overmix — just combine until ingredients are evenly distributed. Overmixing makes the meatloaf dense.
Step 3: Shape the Meatloaf : Transfer the mixture onto the prepared baking sheet. Shape it into a loaf about 8 inches long and 4 inches wide. If using a loaf pan, press the mixture evenly into it.
Step 4: Prepare the Glaze : In a small bowl, whisk together ketchup, brown sugar, vinegar, and mustard (if using). Spoon about ⅓ of the glaze over the top of the uncooked meatloaf, spreading it evenly.
Step 5: Bake : Bake in the preheated oven for 45 minutes. Remove the loaf and spread another ⅓ of the glaze on top. Return to oven and bake for another 15–20 minutes, or until: The internal temperature reaches 160°F (70°C). The top is caramelized and glossy.
Step 6: Rest and Serve : Let the meatloaf rest for 10 minutes before slicing. Brush with the remaining glaze before serving for extra shine and flavor.