Preheat your oven to 375°F. Line a baking sheet with parchment paper or lightly grease a 9x5-inch loaf pan.
In a large mixing bowl, combine the breadcrumbs and milk. Let them sit for 5 minutes to absorb. This keeps the meatloaf tender.
Add the eggs, grated onion, garlic, Worcestershire sauce, parsley, salt, and pepper. Stir until well combined.
Add the ground beef. Using clean hands, gently mix everything together. Be careful not to overwork the meat, as that can make it tough.
Shape the mixture into a loaf and place it on the prepared pan (or press into the loaf pan).
In a small bowl, whisk together the ketchup, brown sugar, and vinegar. Spread half the glaze evenly over the meatloaf.
Bake for 40 minutes. Remove from the oven, spread the remaining glaze on top, and bake for another 15–20 minutes, or until the internal temperature reaches 160°F.
Let the meatloaf rest for 10 minutes before slicing. This helps the juices redistribute and keeps it moist.