Melt butter, blend flour, and gradually add chicken broth to make a smooth sauc e. Cook over low heat until thick, stirring constantly. Add seasoning salt and mushroom soup. Add peas and carrots, pimentos and onion. Saute green pepper in butter; add to mixture. Pour into greased casserole. Prepare potatoes accord ing directions on package. Top casserole with mashed potatoes using a pastry tube. Bake at 400� for 20 minutes. Yield : 10 servings.