Par-cook the bacon in a large skillet set over medium heat just until the fat is rendered out, 5-7 minutes. The bacon should be pliable and not brown or crispy at all. Transfer bacon to a paper towel-lined plate for a few minutes, then place it in the bowl of a food processor fitted with the
chopping blade. Pulse the bacon until it is coarsely ground, 30-60 seconds; set aside.
Discard all except 2 tablespoon rendered bacon fat from the skillet and set the pan over medium-low heat. Add the diced onion to the bacon fat and cook, stirring often, until onion is translucent and has softened, about 4-6 minutes. Add minced garlic, stir and cook 30 seconds.
In a large bowl, mix together the cooked onion/garlic mixture, milk, ketchup, Worcestershire Sauce, Dijon mustard, oats, eggs and black pepper. Let mixture rest for 2 minutes.
In a different large bowl, combine the chopped bacon and ground chuck, mix just until combined.
Pour the liquid mixture over the meat mixture and mix just until ingredients are incorporated. Try not to over-work the mixture or the meatloaf will be a little tough.
Divide the mixture into two equal sections and shape each half into a loaf approximately 8x5x3-inches. Set the loaves on the prepared rack (set over the baking sheeand reshape, if needed.
Pour half the topping mixture over each meatloaf. Cook at 350°F for 50-65 minutes or until the internal temperature reads 160°F, on an instant-read meat thermometer, and the juices run clear. Note – If the topping is browning too quickly after 25 minutes, tent the loaves with foil (don’t wrap tightlto prevent burning.
Transfer loaves to a cooling rack and let rest 10-15 minutes before slicing.