Begin by peeling the celeriac, I use a large knife for this as a peeler just isn't adequate for this job. Top and tail with a large sharp knife and then continue around it by slicing off the skin. Once peeled cut into medium dice. Peel and dice the potato, chop the onion into medium dice. Peel the apples and cut one into medium dice for the soup, reserve the other to garnish.
Have a large saucepan ready and add the olive oil, saute the onion with the bay leaf until translucent. Add the celeriac, potato and apple, stir to coat in the oil and cook for 2-3 minutes.
Add the stock, season with salt and pepper and bring to a simmer, cover with a lid and cook for 15-20 minutes.
In the meantime fry the pancetta in a dry frying pan until golden and the fat has rendered. Tip the pancetta onto a plate lined with kitchen paper. Keep warm. Wipe the pan out with kitchen paper and add a tablespoon of sunflower oil. Peel and cut the remaining apple into fine dice and add to the frying pan. Cook until the apples have softened. Now add the spices and fry on medium for a minute more to release the flavour. Set to one side.
After 20 minutes check the soup to see that the vegetables are really soft, remove the bay leaf and blitz in a food processor until really smooth. Check the seasoning and pour into a clean saucepan to re-heat.
Pour into bowls and top with either creme fraiche of yogurt and a sprinkling of spiced apple and pancetta.