In a 3 quarts casserole dish melt butter, add rice, pimento and chopped bell pepper and mushrooms. Lay chicken pieces on rice. Spoon mushroom soup over chicken. Sprinkle onion soup mix over all. Add 1 1/2 cups water. Cover with foil. Bake at 1 1/2 hours at 325 degrees. Remove foil for the last 15 minutes to let chicken brown.