Wash and peel the celeriac, reserving the peel. Rough chop 500g/1lb 2oz of the flesh and cut the rest into very small dice, about the size of grains of rice.
Place the celeriac peel and 500ml/18fl oz water into a saucepan, bring to the boil and simmer for 30 minutes. Strain through a fine sieve and reserve the stock.
Bring a pan of salted water to the boil, add the roughly chopped celeriac and simmer for 10-12 minutes until soft. Drain, then place into a food processor with 50ml/2fl oz of the white wine vinegar. Blend to a fine purée then pass through a fine sieve.
Cook the winkles in boiling salted water with the teaspoon of white wine vinegar for one minute. Drain the winkles then pick them from the shell and remove the waste sack and foot.
Place the winkles into a saucepan and cover with the chicken stock. Bring to a simmer and cook for 45 minutes.
Heat a non-stick frying pan until medium hot, add the butter and celeriac dice and cook gently for one minute. Add a little of the celeriac stock at a time, as if making a risotto, and when the stock is absorbed add some more, until the celeriac is just tender, this should take only 5-6 minutes. Add the truffle and cook for one minute.
Once cooked, add the mascarpone, parmesan and reduced dark chicken stock. Add the winkles and stir through.
To serve, spoon the celeriac purée into the base of a soup plate. Spoon the 'risotto' over the top. Scatter the pomelo segments over and finish with a little celery leaf.