Thinly slice 1-1/2 onions; finely chop remaining onion half. In a large bowl, toss bread crumbs with chopped onion and seasonings. Add beef; mix lightly but thoroughly. Shape into four 1/2-in.-thick oval patties., Place a lightly oiled large nonstick skillet over medium heat. Add patties; cook 5-6 minutes on each side or until a thermometer reads 160°. Remove from pan. Discard drippings from pan., In a small bowl, mix cornstarch, bouillon and water until smooth. In same skillet, heat butter over medium-high heat. Add mushrooms and sliced onions; cook and stir 5-7 minutes or until onions are tender., Stir in cornstarch mixture. Bring to a boil; cook and stir 1-2 minutes or until thickened. Return Salisbury steaks to pan, turning to coat with sauce; heat through.