Shred the spinach, chop the onion, and fry lightly in the melted butter for 10 min. Boil the stock and pour over the spinach and onion; simmer for 10 min. Rub through a hair or nylon sieve, add the milk to the purée. Measure the soup and thicken with ½ oz. cornflour to each 1 pt. of soup, blended with a little cold milk, stock or water. Stir the cornflour paste into the soup, reheat and season to taste, colour if liked. If either cream or egg yolk is used, stir in below boiling point, reheat until egg is cooked but do not allow to boil.