Freeze the soup at the end of step 3 for up to 2 months, or make up to a day ahead and chill until needed.
Put all the butter beans in a large bowl and cover with plenty of cold water so they have room to swell up. Leave to soak overnight.
Drain the beans. Shuck off and discard the skins by pinching the beans between finger and thumb – it’s a laborious job, but worth it. Put the beans in a colander, rinse and shake off the excess water.
Heat the oil in a pan and fry the onion and garlic for 1-2 minutes. Tip in the carrots and celery and fry gently for 2-3 minutes. Add the beans, stock, bay and thyme with some pepper, and bring to the boil. Reduce the heat, cover and simmer for 20-25 minutes, skimming off any scum, until the beans are soft. Cool for 10 minutes, discard the bay and thyme, then whizz the soup in a food processor until smooth. (You will probably have to do this in batches.)
Return the soup to the rinsed pan and bring to the boil. Check for seasoning, then serve with a drizzle of chilli oil and a sprinkling of thyme leaves.