Preheat oven to 350°F. Line a rimmed baking sheet with foil.
Whisk milk, 1/3 cup ketchup, Worcestershire, and onion soup mix in a large bowl.
Stir in breadcrumbs and let the mixture soak for 2 minutes.
Add ground beef, garlic powder, pepper, salt, and parsley. Mix gently until just combined.
Shape into a loaf about 9x4 inches and place on the prepared sheet.
Combine remaining 1/3 cup ketchup, brown sugar, mustard, and vinegar to make the glaze.
Brush half the glaze over the loaf. Bake for 40 minutes.
Brush with remaining glaze and bake 15 minutes more, until the center reaches 160°F.
Rest 10 minutes before slicing. Serve warm.