Heat the oil in a pan. Sauté the leek, then add the celeriac. Pour in the stock and bring to the boil. Reduce the heat, simmer for approx. 20 mins., puree. Add the seeds, heat the soup, season.
Bring the beetroot to the boil along with the sugar and 50 ml of water, puree. On a baking tray lined with baking paper, spread the mixture into 4 rows approx. 2 mm thick (approx. 5 x 20 cm).
Bake for approx. 2 hrs. in the centre of an oven preheated to 100°C. Immediately remove the beetroot mixture from the baking paper and leave to cool.