Preheat the oven to 350 degrees. In a large bowl, mix the ground beef, breadcrumbs, egg, milk, onion, salt, and pepper until evenly combined.
Shape the mixture into a loaf and place it in a loaf pan. Stir together the maple syrup and brown sugar, then spread half over the top of the loaf.
Bake the meatloaf for 45 minutes. Add the remaining maple brown sugar mixture to the top and bake 15 minutes more until the internal temperature reaches 160 degrees.
While the meatloaf bakes, place the cubed potatoes in a pot of salted water. Bring to a boil and cook until tender, about 15 minutes. Drain and mash with the butter, plus salt and pepper to taste.
In a separate bowl, combine the shredded cabbage, sliced apple, mayonnaise, apple cider vinegar, salt, and pepper. Toss until the slaw is lightly coated.
Slice the meatloaf and serve portions with a scoop of mashed potatoes and a side of apple cabbage slaw.