Spray the bottom of a large saucepan about 7 to 8 times. Turn the heat on high underneath, then when the oil starts to bubble, turn the heat lower.
Add the onion and the celery and sauté, stirring constantly for about 7 to 8 minutes until soft, but not burnt, or you will taint the taste of the soup.
Add the dried thyme and stir in, then add the potato and apple and sauté, again stirring for a further 2 minutes.
Add the stock, bring the mixture to the boil and simmer for about 30 minutes until the vegetables are soft then remove from the heat.
Either using a stick blender or an upright blender, blitz the soup to a pulp or purée and return it to the saucepan.