Trim the celery and chop into thin slices. Peel the onion and finely chop. Peel the garlic cloves and crush or finely chop. Scrub the carrot and finely slice. Peel and dice the potatoes.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped celery, onion, garlic and carrot. Pick the thyme leaves and stir them into the veg. Cook for 7-8 mins, till softened.
Stir the potatoes into the pan and pour in 750ml boiling water. Crumble in the stock cube and stir well to combine. Bring to the boil, then simmer for 15 mins, till the potatoes are tender.
Use a stick blender to whizz the contents of the pan into a smooth soup. No stick blender? No problem. Simply pour the soup into a food processor (in batches if necessary) and blend. Have a taste of the soup and add more seasoning, then ladle into bowls and serve.