an 18-by-13-inch sheet pan or two 9-by-13-inch sheet pans (see Cook's Note)
Position a rack in the upper third of the oven and preheat to 375 degrees F. Mix 1/2 cup of the ketchup, the breadcrumbs, milk, eggs, parsley, and 1 tablespoon Worcestershire sauce together in a large bowl and set aside until the breadcrumbs have absorbed the liquid, about 3 minutes.
Add the ground beef and pork, onion, garlic, 1 tablespoon salt, and 1/2 teaspoon pepper to the bowl and mix until well combined (do not overmix).
Transfer the meat mixture to an 18-by-13-inch sheet pan (see Cook's Note) and press it into an even layer covering the entire surface. Whisk the remaining 1 1/2 cups ketchup, 1 tablespoon Worcestershire, brown sugar, cider vinegar, and chipotle powder in a small bowl. Brush the meatloaf with the sauce.
Bake until the meat is cooked through, about 20 minutes, then turn on the broiler setting and broil until the top becomes caramelized and dark in spots, 3 to 4 minutes more, turning the pan periodically for even broiling. Let rest 5 minutes before serving.
If using two 9-by-13-inch sheet pans, bake at 425 degrees F until the meat is cooked through, 10 to 12 minutes, then broil.
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