In a small bowl, combine the onion, bread crumbs, milk, 1/2 teaspoon salt and pepper. Crumble beef over mixture and mix lightly but thoroughly. Shape into 2 patties. Place in a greased 11x7-in. baking dish. , Combine the water, brown sugar, ketchup, mustard, vinegar and remaining salt; pour over patties. , Bake, uncovered, at 350° until a thermometer reads 160°, 35-40 minutes. If desired, serve on buns with red onion.