Cut the vegetables - carrots, mushrooms, the white part of leek and celery-into julienne strips, 2 mm (1/16 in) wide by 4 cm (1½ in) long. In the case of carrots and celeriac, it is quicker and easier to use a mouli-julienne. Mushrooms have more pliable flesh and should be sliced with a small knife.
Bring the chicken stock to the boil and season with salt and p