In a medium saucepan, combine water, chicken livers and sliced onion. Bring to the boil, reduce heat to low, and cover. Simmer for about 20 minutes, or until liver is cooked and tender. Remove from heat, drain, and discard onions. Also remove and discard any hard portions of the liver.
Place cooked livers in a blender or food processor, and process until smooth. Add chopped onion, sherry, butter, salt, black pepper and mace; pulse to blend. Spoon pate mixture into a bowl or dish; chill for 1 hour before serving.