Slice up the onions and fry them in the butter, strain them out and return the butter to the saucepan. Stir in the cornflour, and when well mixed pour over the stock and stir until it boils. Slice the kidney up into small pieces, and put it in; simmer very gently for one hour. Just before serving, season with salt and a little lemon juice; pour into a tureen and sprinkle a little chopped parsley on top.
This soup must be cooked very slowly, or the kidney will be hard and tough.
Time: 1 hour.