1. Set the stock to heat to boiling. Stir the juice of 1 lemon into a large bowl of cold water.
2. Trim, wash and peel the scorzonera one root at a time. As each root is peeled, cut it lengthwise into thick slices; cut each of these into 1 1/2 in (4 cm) lengths and remove any of the hard, woody central section from each piece with a sharp knife.
3. Drop these into the bowl of acidulated water to prevent them discolouring.
4. Chop the onion finely. Heat the oil in a large, heavy bottomed saucepan and fry the onion gently with the chopped parsley for a few minutes, until tender.
5. Drain the scorzonera well, add to the pan and saute for 5 minutes. Add a pinch of salt, sprinkle with the flour and continue stirring for 1 minute to cook the flour and distribute it evenly.
6. Add approx. 16 fl oz (450 ml) of the boiling hot stock and stir well. Reduce the heat to low, cover and simmer for 10 minutes, adding a little more hot stock when needed.
7. When the scorzonera is tender, turn down the heat as low as possible and make a thickening liaison by beating the egg yolks with the juice of the second lemon and 2 tbsp of the hot stock in a small bowl.
8. Sprinkle over the scorzonera and continue cooking, stirring continuously, over a very low heat for 1 - 2 minutes. The egg yolk will thicken the sauce a little and make it slightly creamy; as soon as it does, remove from the heat.
9. This is not meant to be a thick sauce. Continue stirring for half a minute off the heat, correct the seasoning and serve without delay.