Preheat oven to 375 degrees F (190 degrees C).

Pour milk over breadcrumbs in a large bowl; soak until crumbs are softened, at least 5 minutes.

Mix ground beef, egg, onion, salt, and black pepper with the bread crumbs and milk. Shape the beef mixture into a loaf and transfer to a baking dish.

Stir ketchup, brown sugar, mustard, and nutmeg together in a bowl until smooth; spread over the loaf.

Bake in the preheated oven until no longer pink in the center, about 1 hour. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).