Position a rack in the upper third of the oven; preheat to 400 degrees . Line a baking sheet with foil; coat with cooking spray. Using a food processor, pulse the bread into crumbs; in a bowl, combine with the milk. Let sit for 5 minutes.
Combine the breadcrumbs, meatloaf mix, onion, garlic, egg, 1 tsp. salt and 1/2 tsp. pepper. Shape into a 5-by-8-inch rectangle on the sheet.
In a bowl, stir together the ketchup and brown sugar; spoon half of the mixture over the loaf, spreading to cover. Bake, coating with the remaining glaze halfway through, until a thermometer inserted in the center registers 160 degrees , 40 minutes. Let rest for 10 minutes before slicing.