Melt the butter in a large pan over medium heat. Add the leeks and cook until softened, about 5-7 minutes.

Add the celeriac and thyme to the pan and cook for another 5 minutes, stirring occasionally.

Pour in the vegetable stock and bring to a boil. Reduce heat and simmer for 20-25 minutes, or until the celeriac is tender.

Carefully blend the soup with a hand blender or in a blender until smooth.

Stir in the double cream and season with salt and pepper to taste.

Serve hot.