Old-Fashioned Salisbury Steak with Shiitake Mushroom Gravy

1 pound extra-lean ground beef (5-percent or less fat) 1/2 teaspoon salt 2 slices light whole-wheat sandwich bread, torn into small pieces 1 onion, finely chopped 2 teaspoons olive oil One 8-ounce package shiitake mushrooms, stemmed and sliced 1 tablespoon all-purpose flour 1 cup reduced-sodium beef broth 1 teaspoon Worcestershire sauce