Preheat the oven to 350°F (175°C) and line a baking sheet with foil. Set aside.
In a large mixing bowl, add the ground beef.
To prepare the bread crumbs, remove the crust from the bread slices, break the bread into pieces, and pulse in a blender or food processor until the bread is finely crumbled.
Add the homemade bread crumbs, onions, egg whites, ketchup, mustard, Worcestershire sauce, salt, and freshly ground pepper to the ground beef. Mix everything together by hand until well combined.
Shape the mixture into a loaf by hand and place it on the prepared baking sheet. The loaf should measure about 12 inches by 4 inches.
In a small bowl, combine the topping ingredients: ketchup, mustard, and brown sugar. Mix well, then spread evenly over the top of the meatloaf.
Bake for 55 minutes, then remove from the oven and let rest for at least 10 minutes before slicing.
Cut the meatloaf into 10 slices. This recipe yields 5 servings, with 2 slices per serving.