Melt butter in heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add chicken broth; cook over medium heat until mixture is thickened and bubbly, stirring constantly. Stir in salt and pepper. Stir in chicken and next 3 ingredients. Spoon mixture into a lightly greased 11- x 7-inch baking dish.