Chop the onion.

In a small bowl, combine the chopped onion, bread crumbs, milk, 1/2 teaspoon salt and pepper.

Crumble ground beef over the mixture and mix well.

Shape the mixture into two patties.

Place the patties in a greased 11x7-inch baking dish.

In a separate bowl, combine the water, brown sugar, ketchup, ground mustard, white vinegar, and remaining salt.

Pour the sauce over the patties in the baking dish.

Bake, uncovered, at 350°F (175°C) for 35-40 minutes, or until the meat is no longer pink.

Serve the baked hamburgers on buns, if desired.