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, ground pork, ground beef, 1 grated onion, garlic, egg, 1 tsp. salt and 1/2 tsp. pepper. Shape into a 5-by-8-inch rectangle on the sheet.
Bake, spreading a 1/2-inch layer of mashed potatoes on halfway through, until a thermometer inserted in the center registers 160 degrees , 40 minutes. Let rest for 10 minutes before slicing.
In saucepan, caramelize 2 chopped onions; add apples, brown sugar, ginger, allspice, and zest and juice of 1/2 lemon. Simmer, 30 minutes; spoon onto meatloaf before slicing.