Finely chopped fresh Italian (flat-leaf) parsley, if desired
4 cups uncooked medium egg noodles
1/2 cup sour cream
32 oz. beef-flavored broth
1/2 cup finely chopped onion
8 oz. white mushrooms, thinly sliced
4 tbsp. butter
1 tbsp. steak seasoning
1 1/4 pounds boneless beef sirloin steak, cut into thin strips