Heat oil in dutch oven. Grind vegetables and seasonings until pulpy not liquified. Add mixture to heated oil. Add tomato paste, mix thoroughly and simmer. Lightly salt fish and add to mixture. Increase heat to medium and do not stir. Mix corn starch with 1/4 cup of water until smooth. Add to pot. Cook 20-25 minutes. Add butter in last 5 minutes of cooking. Serve over rice or fufu.