Saute onion in butter until soft in a kettle. Add potatoes, salt, basil, pepper and water; cover and simmer 15 minutes. Place fish on top of potatoes; cover and simmer 15 minutes or until fish flakes easily and potatoes are tender. Stir in corn and liquid and evaporated milk. Cover and heat just to boiling. You may sprinkle this with chopped parsley and pimento, if desired. Serves 6.