Peel and thinly slice the onion. Peel and dice the celery. Heat oil in a pot and sauté celery for about 3 minutes, add onion strips. Season with salt and pepper, then add stock and cream and simmer covered for about 20 minutes. Peel and quarter the apple and remove the core. Dice quarter. Add apple cubes after about half the cooking time
Place pancetta slices on a baking tray lined with baking paper. Cover with another layer of baking paper and place a second baking tray on top. Bake the slices in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 5-8 minutes. Remove upper baking tray and baking paper and bake for another 3-4 minutes until golden brown
Finely puree the soup. Wash the chervil, shake dry and chop the leaves of 2 stems very finely. Add chopped chervil to the soup, season with salt and pepper. Arrange the soup and pancetta slices and garnish with chervil