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.