Prep Oven and Pan: Preheat oven to 350°F (175°C). Line a baking sheet with foil or lightly grease a loaf pan.
Cook Onions: In a small skillet, sauté onions over medium heat for 5–7 minutes until soft. Add garlic and cook 1 more minute. Let cool slightly.
Mix Meatloaf: In a large bowl, combine ground beef, eggs, milk, breadcrumbs, cooked onions and garlic, ketchup, Worcestershire sauce, Italian seasoning, parsley, salt, and pepper. Mix gently until just combined.
Shape and Bake: Form the mixture into a loaf shape (8x4 inches) and place on the prepared pan. Bake for 40 minutes.
Prepare Glaze: In a small bowl, whisk together ketchup, brown sugar, Worcestershire sauce, and vinegar.
Glaze the Meatloaf: Spread half of the glaze over the meatloaf and return to the oven for 20–25 minutes, or until the internal temperature reaches 160°F (71°C).
Rest and Serve: Let the meatloaf rest for 10 minutes before slicing. Spread remaining glaze on top if desired. Serve warm with mashed potatoes, roasted vegetables, or a side salad.