Slice or chop the main and flavouring vegetables. Melt the fat in a deep pan and in it cook the vegetables over gentle heat for 10 min. Keep the lid on the pan and shake it vigorously from time to time. Boil the stock, add it to the vegetables with the herbs and other flavouring (if used) and simmer the whole until the vegetables are quite soft. This cooking time should be as short as possible but will vary with the kind and age of the vegetables used. Remove the herbs, rub or press the vegetables through a sieve (wire for soft, pulpy or very firm vegetables; nylon if a very smooth purée is needed). Mix the liquid with the purée and measure the soup. Weigh or measure the thickening in the proportion given above. Blend the starch powder with cold milk, stock or water and stir it into the soup. Cook the soup until the starch is thickened and no longer tastes raw. Season carefully to taste.
After the starch thickening has been cooked, remove the pan from the heat. Mix the egg yolk and cream together, stir them into the soup, which should be well below boiling-point. Stir over gentle heat till the egg yolk thickens, but do not boil. Serve the soup at once; cream and eggs cannot be kept hot. Cream when used alone may be stirred into the soup just at boiling-point, as it is removed from the heat. It must not be allowed to boil.
Serve separately with any vegetable purée, fried croûtons of bread, pulled bread, Melba toast or “fairy” toast.