Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large mixing bowl, combine the ground beef, chopped onion, parsley, bread crumbs, milk, Worcestershire sauce, eggs, garlic powder, salt, and pepper. Mix until just combined.
Step 3: Transfer the meat mixture to a meatloaf pan and shape it into a loaf.
Step 4: In a small bowl, mix together the ketchup, balsamic vinegar, and brown sugar to make the glaze.
Step 5: Spread half of the glaze over the top of the meatloaf.
Step 6: Bake for about 1 hour, checking for an internal temperature of 160°F (70°C).
Step 7: After 1 hour, spread the remaining glaze on top and bake for an additional 10-15 minutes.
Step 8: Let the meatloaf rest for about 10 minutes before slicing.
Step 9: Slice and serve with your favorite sides.