Preheat your oven to 350 degrees Fahrenheit. Line a large rimmed baking pan with aluminum foil and spray it thoroughly with cooking spray to ensure the proteins do not bond to the foil.

In a small pan, cook the diced onions in butter over medium-low heat until translucent and tender with a pale golden color. Let them cool completely before adding them to the meat mixture.

In a medium bowl, combine the eggs, milk, and breadcrumbs. Stir them until they form a consistent slurry and let the mixture sit for 5-10 minutes to allow the starch to begin absorbing the liquid.

Add the 4 pounds of lean ground beef, the cooled onions, the 2 tablespoons of ketchup, Italian seasoning, parsley, salt, and pepper to the bowl. Use your hands to mix until the ingredients are just combined; avoid overworking the meat.

Form the mixture into a large free-form loaf on the prepared baking pan, aiming for a shape approximately 10 to 12 inches long and 5 inches wide to ensure even heat penetration. Bake for an initial 75 to 90 minutes.

While the meatloaf is performing its initial bake, whisk together the chili sauce, ketchup, and brown sugar in a small bowl until the sugar is dissolved and the sauce is smooth.

Once the initial bake time is complete, remove the pan from the oven. Spread the sauce mixture generously over the top and sides of the loaf.

Return it to the oven and bake for an additional 15 to 30 minutes. You are finished when the center reaches an internal temperature of 160 degrees Fahrenheit.

If you want a darker, more caramelized finish, you can broil the loaf for 1 to 2 minutes at the very end.

Remove the pan from the oven and let the meatloaf rest for at least 10 minutes before slicing.