Preheat the oven to 350 degrees Fahrenheit. Lightly spray a 9 x 5-inch loaf pan with non-stick cooking spray.
In a large bowl combine 2 pounds lean ground beef, 1 small chopped onion, 3 tablespoons chopped fresh parsley, ¾ cup Italian bread crumbs, ½ cup milk, 1 tablespoon Worcestershire sauce, 2 large eggs, 1 teaspoon garlic powder, and salt and pepper.
Use your hands to mix the ingredients gently until just combined, being careful not to overmix.
Press the meat mixture evenly into the prepared loaf pan.
In a small bowl whisk together ¼ cup ketchup, 1 tablespoon balsamic vinegar, and 1 tablespoon brown sugar.
Brush half of the glaze evenly over the top of the meatloaf and reserve the remaining half.
Bake the meatloaf in the preheated oven for 50 minutes.
Remove the meatloaf, brush the remaining glaze on top, then return it to the oven and bake an additional 10 minutes, or until an instant-read thermometer inserted into the center reads 160 degrees Fahrenheit.
Let the meatloaf rest for about 10 minutes before slicing and serving.