Heat oven to 325°F. In ungreased 12x8-inch (2-quart) baking dish, combine mushrooms and onion; spread evenly in bottom of baking dish.
Remove giblets and neck from chicken body cavity. Rinse chicken inside and out with cold water; drain. Pat dry with paper towels. Rub inside of chicken cavity with salt.
In small bowl, combine butter, parsley, rosemary and garlic; mix well. Spread over chicken and inside cavity.
Close neck cavity by bringing loose skin over opening and holding in place with metal skewer. Twist wing tips under back. With string, tie legs and tail together. Place in baking dish over mushrooms and onion.
Bake at 325°F. for 2 to 2 1/2 hours. Chicken is done when drumstick moves easily up and down and twists in socket, and when chicken is fork-tender and juices run clear.
Remove chicken from baking dish; place on serving platter. Cover; let stand 5 to 10 minutes before serving.
Pour mushrooms, onion and pan drippings into 2-cup measuring cup. Remove as much fat as possible from top. Add enough water or chicken broth to equal 1 1/2 cups. Pour into medium saucepan. Bring to a boil. In small bowl, combine 1/4 cup water and flour; blend well. Stir into boiling mixture; cook until thickened. If desired, add salt and pepper to taste.