Reduce the stock by one third.
When they are slightly coloured add the onion celery and leek and cook until soft and golden. Add the flour stirring well and cook for a fiurther 10-15 minutes.
Lastly add the herbs.
Pass the demi glace through a double muslin and leave to cool.
Store in the refrigerator.
Strictly speaking veal bones are the main ingredients for a stock to convert to a demiglace. Chicken bones give a lighter result however which is why I much prefer using a chicken stock as the basis for our demiglace. I also include a little flour as it absorbs and retains the truffle and madeira flavours indeed all flavours so well when added later.