Preheat your oven to 375°F (190°C). In a large bowl, combine ground beef, egg, milk, breadcrumbs, onion, garlic, Worcestershire sauce, salt, and pepper. Mix until just combined—avoid overmixing. Shape the mixture into a loaf and place it into a loaf pan or on a foil-lined baking sheet. In a small bowl, whisk together ketchup, brown sugar, Dijon mustard, and vinegar to create the glaze. Spread half of the glaze over the meatloaf. Bake uncovered for 40 minutes. Remove from oven, brush the remaining glaze on top, and bake for another 5–10 minutes or until the internal temperature reaches 160°F. Let the meatloaf rest for 5 minutes before slicing and serving.