Start by pre-heating the oven to 350 °F (175 °C) and lining a rimmed baking sheet. Sauté the chopped onion in olive oil over medium heat for about 5-7 minutes until softened and lightly golden. Let it cool slightly while preparing the rest.
In a large bowl, combine the ground beef, sautéed onion, minced garlic, parsley, breadcrumbs, milk, eggs, ketchup, salt, pepper, and Italian seasoning. Mix gently with your hands until just combined—over-mixing makes it dense.
Shape the mixture into a loaf about 8 inches long, 4 inches wide, and roughly 3 inches tall on the lined baking sheet. Bake uncovered at 350 °F for about 40 minutes.
While the loaf bakes, whisk together the glaze ingredients: ketchup, white vinegar, brown sugar, garlic powder, and onion powder. After the first 40 minutes, spread the glaze evenly over the top of the meatloaf, then return it to the oven for another 20 minutes or until the internal temperature reaches 160 °F.
Once baked, let the meatloaf rest for 10-15 minutes before slicing. This helps the juices redistribute, keeping every slice moist and flavorful.