Heat some olive oil in a pan and add the onion, stir and fry gently until the onion softens but doesn’t brown. Add the bay leaf and stock and bring to a simmer. Peel and chop the celeriac working as fast as you can, and putting each bit you have chopped straight into the hot stock so it doesn’t discolour. Simmer until the celeriac is tender, about 15 minutes, then add the cream, take out the bay leaf and purée the soup with a stick blender. Pour it through a sieve into a clean pan, bring back to a simmer and add the cheddar. Take off the heat.