Shred one of the onion halves using the large holes of a box grater.
Using a few paper towels, press against the grated onion, to absorb most of the onion juice, and reserve the onions.
Thinly slice the other onion half, and reserve.
Mix the 1/2-cup ketchup. 1/4-cup brown sugar, and the Worcestershire, in a small bowl, and reserve.
In a large bowl, combine the ground sirloin, breadcrumbs, mustard, sweet relish, whole milk, shredded onion, egg and 4 tablespoons of ketchup. Mix until just combined. Do not over mix.
Chef’s Note: The best tools of the chef are his/her hands… mix with your hands, but don’t over do it.
Chef's Tip: Why Panko Breadcrumbs? Traditional breadcrumbs, unless you're making a panade, will absorb too much of the liquid and make for a much dryer meatloaf. Panko breadcrumbs will not absorb much liquid, and therefore create a juicer meatloaf with a distinct mouthfeel.
Place the meatloaf mixture into a loaf-size pan.
Chef’s Tip: My favorite pan for this type of baking is the type that has the insert that raises the meatloaf off the bottom of the pan. This keeps the meatloaf from cooking in its own grease.
In a skillet, add the 1-tablespoon of oil, and heat over medium high heat until the oil is shimmering. Add the sliced onions, and the 2 tablespoons of brown sugar.
Cook onions until softened, and just beginning to brown, about 5 to 8 minutes.
Place rack in bottom third of oven, and preheat to 355f (180c).
Spread the ketchup/brown sugar/ Worcestershire mix evenly over the top of the meatloaf.
Add the cooked onions to the top, and cover the pan with a sheet of aluminum foil.
Bake for 35 minutes, in the preheated oven, and then remove the foil.
Increase the temperature to 375f (190c), and bake an additional 25 minutes.
Chef's Note: The internal temperature of the meatloaf should be 155f (68c). Once removed from the oven, and allowed to rest, the internal temperature will raise to 160f (71c), which is safe for beef products. If you allow the temperature to rise much above this recommendation, it will begin to dry out the meatloaf.
Serving Tip: To make this dish even homier, I like to serve it with some rustic smashed potatoes, and maybe some green beans… enjoy.