Make the dressing by combining the soured cream and vinegar with some seasoning.
Get a large bowl of iced water ready. Break the celery down into sticks and set aside any of the tender leaves from the middle. Using a swivel blade peeler, peel each stick of celery, discarding the first stringy layer. Continue to make strings or ribbons and tip these into the iced water until needed.
Cut the apple into thin slices, discarding the core, then cut the slices into thin matchsticks.
Drain the celery and toss in a large bowl with the dressing, apple and most of the cheese and walnuts.
Spread the salad onto a platter and scatter with the remaining walnuts, blue cheese and celery leaves.
